[CodeGenPrepare] Reset the debug location when promoting trunc(s)

The promotion machinery in CGP moves instructions retaining
debug locations. When the transformation is local, this is mostly
correct, but when instructions are moved cross-BBs, this is not
always true and causes jumpiness in line tables. This is the first
of a series of commits. sext(s) and zext(s) need to be treated
similarly.

Differential Revision:  https://reviews.llvm.org/D81879