Fix RyuJIT assert 'emitCurIG != emitPrologIG'

Fix RyuJIT assert 'emitCurIG != emitPrologIG'

This occurs when the call-to-finally block exists as the last code in the function
(including any funclets). After we generate all "normal" code, we start over and
generate code for all the prologs and epilogs. In this case, we didn't reset the
emitForceNewIG global, which determines if we should force a new IG to be created
the next time code is generated. It is used for specifying no-GC regions. This fix
initializes it (along with emitNOGCIG) at the beginning of generating and prolog
or epilog.