Function specialization was crashing on poison values and constexpr values.
The problem is that these values are not added to the solver, so it crashes
when a lookup is performed for these values. This fixes that by not
specialising on these values. For poison that is obvious, but for constexpr
this is a change in behaviour. Thus, in one way this is a bit of a stopgap, but
specialising on constexpr values wasn't done very intentionally, and need some
more work and tests if we wanted to support this.
As a follow up, we need to look if the solver should exit more gracefully and
return a "don't know", or that it should really support these constexprs.
This should fix PR51600 (https://bugs.llvm.org/show_bug.cgi?id=51600).
Differential Revision: https://reviews.llvm.org/D110529
eba76056a3