[Function Specialisation] Fix use after free

This is a fix for a use-after-free found by the address sanitizer when
compiling GCC: https://github.com/llvm/llvm-project/issues/52821

The Function Specialization pass may remove instructions, cached
inside the PredicateBase class, which are later being dereferenced
from the SCCPInstVisitor class. To prevent the dangling references
I am lazily deleting the dead instructions after the Solver has run.

Differential Revision: https://reviews.llvm.org/D118591