Remove a case where we know we don't have a
profiler, and avoid incrementing for a distributed
factory method. Otherwise such cases could pass
a null ASTNode, which will become an assertion
failure in a future commit.
While we're here, let's standardize on emitting
the profiler increment for function entry after
the prolog, as if there's e.g an unreachable
parameter, the increment can be safely elided
anyway.
This is a dedicated instruction for incrementing a
profiler counter, which lowers to the
`llvm.instrprof.increment` intrinsic. This
replaces the builtin instruction that was
previously used, and ensures that its arguments
are statically known. This ensures that SIL
optimization passes do not invalidate the
instruction, fixing some code coverage cases in
`-O`.
rdar://39146527
