[llvm-mca] Fixed a bug in the logic that checks if a memory operation is ready to execute.

Fixes PR37790.

In some (very rare) cases, the LSUnit (Load/Store unit) was wrongly marking a
load (or store) as "ready to execute" effectively bypassing older memory barrier
instructions.

To reproduce this bug, the memory barrier must be the first instruction in the
input assembly sequence, and it doesn't have to perform any register writes.

llvm-svn: 334633