[X86][GISel] Add legalization support for non-constant G_BUILD_VECTOR

[RKSimon]

X86LegalizerInfo::legalizeBuildVector currently only handles constant build vectors.

If we can add initial support for non-constant values - even just as an UNPCK chain - we have a fallback for many vector opcodes (G_CONCAT_VECTORS / G_SHUFFLE_VECTOR / G_INSERT_SUBVECTOR / G_EXTRACT_SUBVECTOR etc.).

[llvmbot]

@llvm/issue-subscribers-backend-x86

Author: Simon Pilgrim (RKSimon)

X86LegalizerInfo::legalizeBuildVector currently only handles constant build vectors.

If we can add initial support for non-constant values - even just as an UNPCK chain - we have a fallback for many vector opcodes (G_CONCAT_VECTORS / G_SHUFFLE_VECTOR / G_INSERT_SUBVECTOR / G_EXTRACT_SUBVECTOR etc.).

[e-kud]

@RKSimon we still have a major blocker regarding indices that stops almost all the vector work #111503 (comment). I try to avoid writing custom c++ selection and use existing patterns. But they can't match because of SDAG and GISel incompatibility in terms of index type.