[TableGen][GlobalISel] Reset the internal map of RuleMatchers just before the emission

Between the creation of the last InstructionMatcher and the first
emission of the related Rule, we need to clear the internal map of IDs.
We used to do that right after the creation of the main
InstructionMatcher when building the rule and although that worked, this
is fragile because if for some reason some later code decides to create
more InstructionMatcher before the final call to emit, then the IDs
would be completely messed up.

Move that to the beginning of "emit" so that the IDs are guarantee to be
consistent.

NFC.

llvm-svn: 321053