Avoid pointless canonicalize when using Dwarf names

CPU profile indicated memcmp was hot due to the two rfind calls in
getCanonicalFnName. If UseSymbolTable is false, we can avoid the cost entirely.

For CSSPGO profiles I've measured ~5% speedup with this change.

Profile similarity before/after matches 100%.

Reviewed By: wenlei

Differential Revision: https://reviews.llvm.org/D151441