Fix some problems in SpeculativelyExecuteBB. Basically,

because of dead code, a phi could use the speculated instruction
that was not in "BB2".  Make this check explicit and tighten up 
some other corners.  This fixes PR3292.  No testcase becauase this
depends entirely on visitation order of blocks and requires a 
sequence of 8 passes to repro.

llvm-svn: 62476