Eliding the vReg to NZCV conversion instruction for G_UADDE/... is illegal if
it causes the carry generating instruction to become dead because ISel
will just remove the dead instruction.
I accidentally introduced this here: https://reviews.llvm.org/D153164.
As far as I can tell, this is not exposed on the default clang settings,
because on O0 there is always a G_AND between boolean defs and uses, so
the optimization doesn't apply. Thus, when I tried to commit
https://reviews.llvm.org/D159140, which removes these G_ANDs on O0, I
broke some UBSan tests.
We fix this by recursively selecting the previous (NZCV-setting) instruction before continuing selection for the current instruction.
b1a6b2cc40