[DAGCombiner] match vector compare and select sizes with extload operand (PR37427)

This patch started off much more general and ambitious, but it's been a nightmare 
seeing all the ways x86 vector codegen can go wrong.

So the code is still structured to allow extending easily, but it's currently 
limited in several ways:

1. Only handle cases with an extending load.
2. Only handle cases with a zero constant compare.
3. Ignore setcc with vector bitmask (SetCCWidth != 1) - so AVX512 should be unaffected.

The motivating case from PR37427:
https://bugs.llvm.org/show_bug.cgi?id=37427
...is the 1st test, and that shows the expected win - we eliminated the unnecessary 
intermediate cast.

There's a clear regression in the last test (sgt_zero_fp_select) because we longer 
recognize a 'SHRUNKBLEND' opportunity. I think that general problem is also present 
in sgt_zero, so I'll try to fix that in a follow-up. We need to match a sign-bit 
setcc from a sign-extended operand and remove it.

Differential Revision: https://reviews.llvm.org/D47330

llvm-svn: 334378