[BOLT][NFC] Improve performance of MCPlusBuilder::initAliases

It was using a redundant iteration over super regs to build
SmallerAliasMap. Removing this results in exactly the same alias maps
and a noticeable performance gain on targets with a large number of
registers.

Just anecdotally: on my machine, processing a small AArch64 binary went
from 2.7s down to 80ms.

Reviewed By: Amir

Differential Revision: https://reviews.llvm.org/D145779