[TableGen] Add `countRendererFns` to `InstructionOperandMatcher`

Without it, the count of renderer functions is inaccurate and, in some
edge cases (like the patterns added in D134354), we can actually
go out of bounds (run out of pre-allocated renderer function spaces
in the GISel state)

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D134861