[X86] Space out the encodings of X86 instruction formats. I plan to add some...

[X86] Space out the encodings of X86 instruction formats. I plan to add some new encodings in future commits and this will reduce the size of those commits. NFC

This tries to keep all the ModRM memory and register forms in their own regions of the encodings. Hoping to make it simple on some of the switch statements that operate on these encodings.

llvm-svn: 279422