In our backend, the successor edges from the blocks only point to where
QPU control flow goes, not where the notional control flow goes from a
"break" or "continue" modifying the execution mask to resume writing to
some channels later.  As a result, this attempt at restricting live ranges
ended up missing the live range of a value where a conditional
break/continue was present in a loop before the later def of a variable.
The previous commit ended up fixing the problem that the flag tried to
solve.

Fixes glsl-vs-loop-continue.shader_test and/or
glsl-vs-loop-redundant-condition.shader_test based on register allocation
results.