Extend array index rewrite to fields

[presenthee]

Related #112

Summary

Handles two limitations of above:

• Currently rewrites only if there is a source variable for the base pointer. To rewrite a broader range of pointers, selection criteria should be improved.
• Currently rewriter only rewrites if base pointer offset is not zero, so some target groups are selected but not rewritten yet.

Changes

• Extend index rewriting to struct field base pointers. (e.g. (*param).field)
• To restrict rewrites which may increase unsafe feature counts, add gates to filter rewrite selections.

Test

Regression test compared to master

feature	master	array-local-rewrite	delta
from_ptr	57	58	+1
from_raw_parts_mut	7530	7528	-2
offset	2698	2695	-3

test case	differences with delta
Public-Tests/B01_organic/flip_horizontal_lib	from_raw_parts_mut: 1 -> 0 (-1), offset: 2 -> 4 (+2)
Public-Tests/B02_organic/cJSON_lib	from_ptr: 2 -> 3 (+1), from_raw_parts: 50 -> 49 (-1)
Public-Tests/B02_organic/hm_geti_lib	offset: 174 -> 167 (-7)
Public-Tests/B02_organic/load_png_mem_lib	offset: 60 -> 63 (+3)
Public-Tests/B02_organic/pinflate_lib	offset: 13 -> 16 (+3)
Public-Tests/B02_organic/sh_geti_lib	offset: 147 -> 140 (-7)
Public-Tests/B02_synthetic/arrayfunc_lib	offset_from: 1 -> 0 (-1)
Public-Tests/B02_synthetic/confusion_lib	from_raw_parts: 0 -> 1 (+1), from_raw_parts_mut: 1 -> 0 (-1), offset: 0 -> 3 (+3), offset_from: 1 -> 2 (+1)

Rewritten test cases (including base pointers stored in fields)

Public-Tests/B01_organic/flip_horizontal_lib
Public-Tests/B01_organic/wcscat_lib
Public-Tests/B02_organic/cJSON_lib
Public-Tests/B02_organic/helxo_lib
Public-Tests/B02_organic/hm_geti_lib
Public-Tests/B02_organic/intput_lib
Public-Tests/B02_organic/load_png_mem_lib
Public-Tests/B02_organic/pinflate_lib
Public-Tests/B02_organic/sh_geti_lib
Public-Tests/B02_organic/sh_puts_lib
Public-Tests/B02_organic/str_dups_lib
Public-Tests/B02_organic/str_put_lib
Public-Tests/B02_synthetic/arrayfunc_lib
Public-Tests/B02_synthetic/confusion_lib
Public-Tests/B02_synthetic/dataentry_lib
Public-Tests/B02_synthetic/memchra2_lib
Public-Tests/P01_sphincs_plus/005_* through Public-Tests/P01_sphincs_plus/132_*

Example transformation

// pinflate_lib
// Before
                let mut src: *const i8 =
                    __crat_borrowed_s.out.offset(-(backwards_distance as
                                    isize));
                let mut dst: *mut i8 = __crat_borrowed_s.out;
                __crat_borrowed_s.out =
                    __crat_borrowed_s.out.offset(length as isize);
                   ...
                    _ => {
                        loop {
                            let fresh0 = length;
                            length = length - 1;
                            if !(fresh0 != 0) { break; }
                            let fresh1 = *src;
                            src = src.offset(1);
                            *dst = fresh1;
                            dst = dst.offset(1);

// After
                let mut src_idx: isize =
                    out_idx + (-(backwards_distance as isize));
                let mut dst_idx: isize = out_idx;
                out_idx = out_idx + (length as isize);
                ...
                    _ => {
                     loop {
                            let fresh0 = length;
                            length = length - 1;
                            if !(fresh0 != 0) { break; }
                            let fresh1 =
                                *(__crat_borrowed_s.out.offset(src_idx) as *mut i8);
                            src_idx = src_idx + 1;
                            *(__crat_borrowed_s.out.offset(dst_idx) as *mut i8) =
                                fresh1;
                            dst_idx = dst_idx + 1;
                        }