X86: Add patterns for X86ISD::VSEXT in registers.

Those can occur when something between the sextload and the store is on the same
chain and blocks isel. Fixes PR14887.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172353 91177308-0d34-0410-b5e6-96231b3b80d8