[instcombine] Add align return attributes for operator new(..., align_val)

(Split from original patch to separate non-NFC part and add coverage.  I typoed when adding the new test, so this change includes the typo fix to let libfunc recongize the signature.  Didn't figure it was worth another separate commit.)

Differential Revision: https://reviews.llvm.org/D116851 (part 2 of 2)