[SCCP] Turn loads of null into undef instead of zero initialized values
Surprisingly, this is a correctness issue: the mmx type exists for
calling convention purposes, LLVM doesn't have a zero representation for
them.
This partially fixes PR23999.
Pull in r241143 from upstream llvm trunk (by David Majnemer):
[LoopUnroll] Use undef for phis with no value live
We would create a phi node with a zero initialized operand instead of
undef in the case where no value was originally available. This was
problematic for x86_mmx which has no null value.
These fix a "Cannot create a null constant of that type!" error when
compiling the graphics/sdl2_gfx port with MMX enabled.
Reported by: amdmi3
5f4899dbfe