[PatternMatch] Match XOR variant of unsigned-add overflow check.

Instcombine folds (a + b <u a) to (a ^ -1 <u b) and that does not match
the expected pattern in CodeGenPerpare via UAddWithOverflow.

This causes a regression over Clang 7 on both X86 and AArch64:
https://gcc.godbolt.org/z/juhXYV

This patch extends UAddWithOverflow to also catch the XOR case, if the
XOR is only used in the ICMP. This covers just a single case, but I'd
like to make sure I am not missing anything before tackling the other
cases.

Reviewers: nikic, RKSimon, lebedev.ri, spatel

Reviewed By: nikic, lebedev.ri

Differential Revision: https://reviews.llvm.org/D74228