[IndVarSimplify][DebugLoc] When widening the exit loop condition, correctly...

[IndVarSimplify][DebugLoc] When widening the exit loop condition, correctly reuse the debug location of the original comparison.

When the loop exit condition is canonicalized as a != compaison, reuse the
debug location of the original (non canonical) comparison.

Before this patch, the debug location of the new icmp was obtained from the
loop latch terminator. This patch fixes the issue by correctly setting the
IRBuilder's "current debug location" to the location of the original compare.

Differential Revision: https://reviews.llvm.org/D25953

llvm-svn: 285185