[MIRVRegNamer] Avoid opcode hash collision

D121929 happens to cause CodeGen/MIR/AArch64/mirnamer.mir to fail due
to a hash collision caused by adding two extra opcodes. The collision
is only in the top 19 bits of the hashed opcode so fix this by just
using the whole hash (in fixed width hex for consistency) instead of
the top 5 decimal digits.

Differential Revision: https://reviews.llvm.org/D137155