[SPIRV] Add legalization for long vectors (#169665)
This patch introduces the necessary infrastructure to legalize vector
operations on vectors that are longer than what the SPIR-V target
supports. For instance, shaders only support vectors up to 4 elements.
The legalization is done by splitting the long vectors into smaller
vectors of a legal size.
Specifically, this patch does the following:
- Introduces `vectorElementCountIsGreaterThan` and
`vectorElementCountIsLessThanOrEqualTo` legality predicates.
- Adds legalization rules for `G_SHUFFLE_VECTOR`,
`G_EXTRACT_VECTOR_ELT`,
`G_BUILD_VECTOR`, `G_CONCAT_VECTORS`, `G_SPLAT_VECTOR`, and
`G_UNMERGE_VALUES`.
- Handles `G_BITCAST` of long vectors by converting them to
`@llvm.spv.bitcast` intrinsics which are then legalized.
- Updates `selectUnmergeValues` to handle extraction of both scalars
and vectors from a larger vector, using `OpCompositeExtract` and
`OpVectorShuffle` respectively.
Fixes https://github.com/llvm/llvm-project/pull/165444
