AsmMatcherEmitter: Add an option to override custom converters for InstAlias

If there is an InstAlias defined for an instruction that had a custom
converter (AsmMatchConverter), then when the alias is matched,
the custom converter will be used rather than the converter generated
by the InstAlias.

This patch adds the UseInstAsmMatchConverter field to the InstAlias
class, which allows you to override this behavior and force the
converter generated by the InstAlias to be used.

This is required for some future improvemnts to the R600 assembler.

Differential Revision: http://reviews.llvm.org/D9083

llvm-svn: 238210