[X86] Minor cleanup to getExtendInVec. NFCI

Use unsigned to calculate the subvector index to avoid a cast.

Remove an unnecessary condition and replace it with a stronger assert.

Use the InVT variable we updated when we extracted instead of grabbing it from the In SDValue.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346983 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed