[X86] Add a hasOneUse check to selectScalarSSELoad to keep the same load from...

[X86] Add a hasOneUse check to selectScalarSSELoad to keep the same load from being folded multiple times.

Summary: When selectScalarSSELoad is looking for a scalar_to_vector of a scalar load, it makes sure the load is only used by the scalar_to_vector. But it doesn't make sure the scalar_to_vector is only used once. This can cause the same load to be folded multiple times. This can be bad for performance. This also causes the chain output to be duplicated, but not connected to anything so chain dependencies will not be satisfied.

Reviewers: RKSimon, zvi, delena, spatel

Subscribers: andreadb, llvm-commits

Differential Revision: https://reviews.llvm.org/D26790

llvm-svn: 287983