[X86] Use (SUBREG_TO_REG (MOV32rm)) for extloadi64i8/extloadi64i16 when the...

[X86] Use (SUBREG_TO_REG (MOV32rm)) for extloadi64i8/extloadi64i16 when the load is 4 byte aligned or better and not volatile.

Summary:
Previously we would use MOVZXrm8/MOVZXrm16, but those are longer encodings.

This is similar to what we do in the loadi32 predicate.

Reviewers: RKSimon, spatel

Reviewed By: RKSimon

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60341

llvm-svn: 357875