[LLVM] Change llvmFor to exclusive stop condition

[mratsim]

See #560

Constantine already uses exclusive stops with staticFor.

[arnetheduck]

Constantine already uses exclusive stops with staticFor.

https://github.com/status-im/nim-stew/blob/28b223eaabba781e3e9db12d01f39504438d9cca/stew/staticfor.nim#L31C48-L31C54 - an option for staticFor is to use a Slice instead which defers the inclusive vs exclusive choice to the language