SPIR-V doesn't currently encode "native" integer bit-widths in its
datalayout(s). This is problematic as it leads to optimisation passes,
such as InstCombine, getting ideas and e.g. shrinking to non
byte-multiple integer types, which is not desirable and can lead to
breakage further down in the toolchain. This patch addresses that by
encoding `i8`, `i16`, `i32` and `i64` as native types for vanilla SPIR-V
(the spec natively supports them), and `i32` and `i64` for AMDGCNSPIRV
(where the hardware targets are known). We also set the stack alignment
on the latter, as it is overaligned (32-bit vs 8-bit).
2c13dec328