[X86][SSE] Add support for combining VZEXT_MOVL target shuffles

[X86][SSE] Add support for combining VZEXT_MOVL target shuffles

Includes adding more general support for the pattern: VZEXT_MOVL(VZEXT_LOAD(ptr)) -> VZEXT_LOAD(ptr)

This has unearthed a couple of latent poor codegen issues (MINSS/MAXSS scalar load folding and MOVDDUP/BROADCAST load folding patterns), which will be fixed shortly.

Its also reduced a couple of tests so that they no longer reach the instruction threshold necessary to be combined to PSHUFB (see PR26183).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@279646 91177308-0d34-0410-b5e6-96231b3b80d8