pattern-matching: use one-phase typechecker
pattern-matching: use one-phase typechecker

Otherwise, during tactic runtime, we run into this bug:

	Impossible: Tm_let and Tm_let

for now, just use the old checker for this.

FStarLang/FStar,A-Manning/FStar,FStarLang/FStar,A-Manning/FStar,FStarLang/FStar,A-Manning/FStar