Fix consume-order checking in codegen.

Fix consume-order checking in codegen.

The switch to LIR invalidated the correspondence between a node's
sequence number and the order in which it must be consumed with respect
to other nodes. This in turn made the consume-order checks during code
generation incorrect.

This change introduces a new field, `gtUseNum`, that is used during code
generation to check the order in which nodes are consumed. Re-enabling
these checks revealed a bug in code generation for locked instructions
on x86, which were consuming their operands out-of-order; this change
also contains a fix for that bug.

Fixes #7963.