[NFC][X86][AArch64] Add tests for the 'check for [no] signed truncation' pattern
As discussed in https://reviews.llvm.org/D49179#1158957 and later,
the IR can be improved:
^ that pattern will be produced by Implicit Integer Truncation sanitizer,
in signed case, therefore it is probably a good idea to improve it.
But as it looks from these tests,
i think we want to revert at least some cases in DAGCombine.
Reviewers: spatel, craig.topper, RKSimon, javed.absar
Reviewed By: spatel
Subscribers: kristof.beyls, llvm-commits
Differential Revision: https://reviews.llvm.org/D49247