[peephole] Enhance folding logic to work for STATEPOINTs

The general idea here is to get enough of the existing restrictions out of the way that the already existing folding logic in foldMemoryOperand can kick in for STATEPOINTs and fold references to immutable stack slots. The key changes are:

    Support for folding multiple operands at once which reference the same load
    Support for folding multiple loads into a single instruction
    Walk all the operands of the instruction for varidic instructions (this is a bug fix!)

Once this lands, I'll post another patch which refactors the TII interface here. There's nothing actually x86 specific about the x86 code used here.

Differential Revision: https://reviews.llvm.org/D24103

llvm-svn: 289510