[SPIR-V] Legalize vector arithmetic and intrinsics for large vectors (#170668)

This patch improves the legalization of vector operations, particularly
focusing on vectors that exceed the maximum supported size (e.g., 4
elements
for shaders). This includes better handling for insert and extract
element
operations, which facilitates the legalization of loads and stores for
long vectors—a common pattern when compiling HLSL matrices with Clang.

Key changes include:
- Adding legalization rules for G_FMA, G_INSERT_VECTOR_ELT, and various
  arithmetic operations to handle splitting of large vectors.
- Updating G_CONCAT_VECTORS and G_SPLAT_VECTOR to be legal for allowed
  types.
- Implementing custom legalization for G_INSERT_VECTOR_ELT using the
  spv_insertelt intrinsic.
- Enhancing SPIRVPostLegalizer to deduce types for arithmetic
instructions
  and vector element intrinsics (spv_insertelt, spv_extractelt).
- Refactoring legalizeIntrinsic to uniformly handle vector legalization
  requirements.

The strategy for insert and extract operations mirrors that of bitcasts:
incoming intrinsics are converted to generic MIR instructions
(G_INSERT_VECTOR_ELT
and G_EXTRACT_VECTOR_ELT) to leverage standard legalization rules (like
splitting).
After legalization, they are converted back to their respective SPIR-V
intrinsics
(spv_insertelt, spv_extractelt) because later passes in the backend
expect these
intrinsics rather than the generic instructions.

This ensures that operations on large vectors (e.g., <16 x float>) are
correctly broken down into legal sub-vectors.