[BOLT] Use opcode name in hashBlock

Use MCInst opcode name instead of opcode value in hashing.

Opcode values are unstable wrt changes to target tablegen definitions,
and we notice that as output mismatches in NFC testing. This makes BOLT YAML
profile tied to a particular LLVM revision which is less portable than
offset-based fdata profile.

Switch to using opcode names which have 1:1 mapping with opcode values for any
given LLVM revision, and are stable wrt modifications to .td files (except of
course modifications to names themselves).

Test Plan:
D150154 is a test commit adding new X86 instruction which shifts opcode values.
With current change, pre-aggregated-perf.test passes in nfc check mode.
Without current change, pre-aggregated-perf.test expectedly fails.

Reviewed By: #bolt, rafauler

Differential Revision: https://reviews.llvm.org/D150005