[Peephole] Allow folding loads into instructions w/multiple uses (such as test64rr)

Peephole opt has a one use limitation which appears to be accidental. The function being used was incorrectly documented as returning whether the def had one *user*, but instead returned true only when there was one *use*. Add a corresponding hasOneNonDbgUser helper, and adjust peephole-opt to use the appropriate one.

All of the actual folding code handles multiple uses within a single instruction. That codepath is well exercised through instruction selection.

Differential Revision: https://reviews.llvm.org/D63656

llvm-svn: 364336