[X86] [AVX512] Minor fix in encoding of scalar EVEX instructions. NFC.

Commit on behalf of Gadi Haber  

Removed EVEX_V512 prefix from scalar EVEX instructions since HW ignores L'L bits anyway (LIG). 4 instructions are modified.
The changed encodings are validated with XED.
Rviewers: delena, igorb

Differential revision: https://reviews.llvm.org/D27802

llvm-svn: 290065