NFC add a new precommit test case for PPCMIpeephole (#90656)

Add pre-commit MIR test for PR "[Promote Pseudo Opcode from 32-bit to
64-bit after eliminating the extsw instruction in PPCMIPeepholes
optimization](#85451)" which
fixes bug reported in the issue "[Inconsistent Output at -O1 and -O2
Optimization Levels on PowerPC64 Due to Complex Type Casting and Nested
Loop Structure](#71030)".

Author: diggerlin