[X86] Remove unneeded isel patterns from VCVTSI2SDZ and VCVTUSI2SDZ. NFC

We had patterns using X86ISD::SCALAR_SINT_TO_FP_RND/SCALAR_UINT_TO_FP_RND for
these instructions. There's nothing to round. Instead, we use a regular
sint_to_fp/uint_to_fp and a movsd as the pattern for these.

llvm-svn: 355796