[X86FixupSetCC] Remember the preceding eflags defining instruction while we're...

[X86FixupSetCC] Remember the preceding eflags defining instruction while we're scanning the basic block instead of looking back for it.

Summary:
We're already scanning forward through the basic block. Might as
well just remember eflags defs instead of doing a bounded search
backwards later.

Based on a comment in D71841.

Reviewers: RKSimon, spatel, uweigand

Reviewed By: uweigand

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71865