[SPIRV] Start adding support for `int128` (#170798)
LLVM has pretty thorough support for `int128`, and it has started seeing
some use. Even thouth we already have support for the
`SPV_ALTERA_arbitrary_precision_integers` extension, the BE was oddly
capping integer width to 64-bits. This patch adds partial support for
lowering 128-bit integers to `OpTypeInt 128`. Some work remains to be
done around legalisation support and validating constant uses (e.g.
cases that get lowered to `OpSpecConstantOp`).
