[X86] Add a new VEX_WPrefix encoding to tag EVEX instruction that have VEX.W==1,...

[X86] Add a new VEX_WPrefix encoding to tag EVEX instruction that have VEX.W==1, but can be converted to their VEX equivalent that uses VEX.W==0.

EVEX makes heavy use of the VEX.W bit to indicate 64-bit element vs 32-bit elements. Many of the VEX instructions were split into 2 versions with different masking granularity.

The EVEX->VEX table generate can collapse the two versions if the VEX version uses is tagged as VEX_WIG. But if the VEX version is instead marked VEX.W==0 we can't combine them because we don't know if there is also a VEX version with VEX.W==1.

This patch adds a new VEX_W1X tag that indicates the EVEX instruction encodes with VEX.W==1, but is safe to convert to a VEX instruction with VEX.W==0.

This allows us to remove a bunch of manual EVEX->VEX table entries. We may want to look into splitting up the VEX_WPrefix field which would simplify the disassembler.

llvm-svn: 335017