[TargetLowering][NFC] More efficient emitPatchpoint().

Current implementation of emitPatchpoint() is very inefficient:
for every FrameIndex operand if creates new MachineInstr with
that operand expanded and all other copied as is.
Since PATCHPOINT/STATEPOINT instructions may have *a lot* of
FrameIndex operands, we end up creating and erasing many
machine instructions. But we can do it in single pass, with only
one new machine instruction generated.

Reviewed By: reames
Differential Revision: https://reviews.llvm.org/D81181