[llvm-mca] Ensure that instructions with a schedule read-advance are always...

[llvm-mca] Ensure that instructions with a schedule read-advance are always issued in the right order.

Normally, the Scheduler prioritizes older instructions over younger instructions
during the instruction issue stage. In one particular case where a dependent
instruction had a schedule read-advance associated to one of the input operands,
this rule was not correctly applied.

This patch fixes the issue and adds a test to verify that we don't regress that
particular case.

llvm-svn: 330032