[X86] Speculatively fix to X86AvoidStoreForwardingBlocks not deference a...

[X86] Speculatively fix to X86AvoidStoreForwardingBlocks not deference a machine mem operand if there isn't one present.

Eric Christopher informed me that FastISel memcpy handling creates
load/store instructions without mem operands. We should fix that,
but I doubt that's the only case of missed mem operands so seems
better to be defensive here.

I don't have a test case yet, but I'll try to add one if i get a
test from Eric.