[X86] Make memcmp vector lowering handle arbitrary expansions

[davezarzycki]

Summary

Teach combineVectorSizedSetCCEquality() to handle arbitrary memcmp expansions but do not change any default policy for now.

This also fixes a bug in the memcmp expansion itself when large displacements are needed.

[davezarzycki]

Updated the tests after running "check-all` with all targets enabled.

[davezarzycki]

Ping. This patch is essentially just bug fixes and a prerequisite change so that we might have a more focused conversation in D69044. Thanks!

[craig.topper]

LGTM

[davezarzycki]

Thanks @craig.topper for suggesting the GEP fix to the memcmp expansion code.

f68925d45083407e6bf322a7ee18d85c83c001ce