[X86][SSE] Add support for 32-bit element vectors to X86ISD::VZEXT_LOAD

Consecutive load matching (EltsFromConsecutiveLoads) currently uses VZEXT_LOAD (load scalar into lowest element and zero uppers) for vXi64 / vXf64 vectors only.

For vXi32 / vXf32 vectors it instead creates a scalar load, SCALAR_TO_VECTOR and finally VZEXT_MOVL (zero upper vector elements), relying on tablegen patterns to match this into an equivalent of VZEXT_LOAD.

This patch adds the VZEXT_LOAD patterns for vXi32 / vXf32 vectors directly and updates EltsFromConsecutiveLoads to use this.

This has proven necessary to allow us to easily make VZEXT_MOVL a full member of the target shuffle set - without this change the call to combineShuffle (which is the main caller of EltsFromConsecutiveLoads) tended to recursively recreate VZEXT_MOVL nodes......

Differential Revision: https://reviews.llvm.org/D23673

llvm-svn: 279619