[LegacyPM] Reduce number of calls to getName

Repeatedly calling getName adds some overhead, which can be easily
avoided by querying the name just once per function. The improvements
are rather small (~0.5% back-end time in a compile-time optimized
setting), but also very easy to achieve.

Note that getting the name should be entirely avoidable in the common
case, but would require more substantial changes.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D148145