nir: Convert a bcsel with only phi node sources to a phi node

nir: Convert a bcsel with only phi node sources to a phi node

v2: Remove the original ALU instruciton after all of its readers are
modified to read the new ALU instruction.

v3: Fix an issue where a bcsel that may not be executed on a loop
iteration due to a break statement is converted to a phi (and therefore
incorrectly "executed").  Noticed by Tim.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=109216
Fixes: 8fb8ebfbb05 ("intel/compiler: More peephole select")
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>