[AMDGPU] Drop verify from SIMemoryLegalizer tests (#78697)

SIMemoryLegalizer tests were slow, with most of them taking 4.5 to 5.3s
to complete and that's on a fast machine. I also recall seeing them in
the slowest tests list on build bots.

This removes the verify-machineinstrs option from these tests to speed
them up, bringing the slowest test down to +-2s.
Verifier still runs in EXPENSIVE_CHECKS builds.