[LLVM][IR] Replace ConstantInt's specialisation of getType() with getIntegerType(). (#75217)

The specialisation will not be valid when ConstantInt gains native
support for vector types.

This is largely a mechanical change but with extra attention paid to constant
folding, InstCombineVectorOps.cpp, LoopFlatten.cpp and Verifier.cpp to
remove the need to call `getIntegerType()`.

Co-authored-by: Nikita Popov <github@npopov.com>