[Analysis] Don't assume that unsigned overflow can't happen in EmitGEPOffset (PR42699)

Summary:
Currently when computing a GEP offset using the function EmitGEPOffset
for the following instruction

  getelementptr inbounds i32, i32* %p, i64 %offs

we get

  mul nuw i64 %offs, 4

Unfortunately we cannot assume that unsigned wrapping won't happen
here because %offs is allowed to be negative.

Making such assumptions can lead to miscompilations: see the new test
test24_neg_offs in InstCombine/icmp.ll. Without the patch InstCombine
would generate the following comparison:

   icmp eq i64 %offs, 4611686018427387902; 0x3ffffffffffffffe

Whereas the correct value to compare with is -2.

This patch replaces the NUW flag with NSW in the multiplication
instructions generated by EmitGEPOffset and adjusts the test suite.

https://bugs.llvm.org/show_bug.cgi?id=42699

Reviewers: chandlerc, craig.topper, ostannard, lebedev.ri, spatel, efriedma, nlopes, aqjune

Reviewed By: lebedev.ri

Subscribers: reames, lebedev.ri, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68342

llvm-svn: 375089