[X86] Allow folding of stack reloads when loading a subreg of the spilled reg

[X86] Allow folding of stack reloads when loading a subreg of the spilled reg

We did not support subregs in InlineSpiller:foldMemoryOperand() because targets
may not deal with them correctly.

This adds a target hook to let the spiller know that a target can handle
subregs, and actually enables it for x86 for the case of stack slot reloads.
This fixes PR30832.

Differential Revision: https://reviews.llvm.org/D26521

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@287792 91177308-0d34-0410-b5e6-96231b3b80d8