[X86] Use binary search of the EVEX->VEX static tables instead of populating two...

[X86] Use binary search of the EVEX->VEX static tables instead of populating two DenseMaps for lookups

Summary:
After r335018, the static tables are guaranteed sorted by the EVEX opcode to convert. We can use this to do a binary search and remove the need for any secondary data structures.

Right now one table is 736 entries and the other is 482 entries. It might make sense to merge the two tables as a follow up. The effort it takes to select the table is probably similar to the extra binary search step it would require for a larger table.

I haven't done any measurements to see if this has any effect on compile time, but I don't imagine that EVEX->VEX conversion is a place we spend a lot of time.

Reviewers: RKSimon, spatel, chandlerc

Reviewed By: RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D48312

llvm-svn: 335092