Save some bits in TSFlags for X86

Code Generation X86
1

Hi, the bits of TSFlags for X86 are running out and the space for instruction formats (in X86BaseInfo.h) is using out too.

So I am going to do the following changes

  1. Combine HasVEX_W with hasREX_W to save 1 bit
  2. Compress CD8_Scale from 7 bits to 3 bits
  3. Extend the FormBits from 7bits to 8bits

Let me know if you have better idea or object to these changes. Thanks.