[CGP] Reset the debug location when promoting zext(s).

When the zext gets promoted, it used to retain the original location,
which pessimizes the debugging experience causing an unexpected
jump in stepping at -Og.

Fixes https://bugs.llvm.org/show_bug.cgi?id=46120 (which also
contains a full C repro).

Differential Revision:  https://reviews.llvm.org/D81437