[X86] Move some hasOneUse checks after checking what the opcode is.

Calling hasOneUse can be expensive on nodes with multiple results.
Especially when some results are Chains. By checking the opcode first,
we can avoid walking the uses if it isn't an interesting node,
and thus avoid calling hasOneUse on a node that might have many uses.

Found by profiling the IR given in D123857.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D123881