When encountering the following SSA graph:
BB1:
#2.T1 [string] = COALESCE #1.CV0($str) [null, string] BB2
BB2:
#5.T1 [string] = QM_ASSIGN string("")
BB3:
#7.X1 [string] = Phi(#2.X1 [string], #5.X1 [string])
FREE #7.T1 [string]
We would currently determine that #7, #5 are dead, and eliminate
the FREE and QM_ASSIGN. However, we cannot eliminate #2, as
COALESCE is also responsible for control flow.
Fix this my marking all non-CV phis as live to start with. This
can be relaxed to check the kind of the source instruction, but
I couldn't immediately come up with a case where it would be
useful.
0826a54836