[AMDGPU][GlobalISel] Update legalization condition for G_BITCAST

[matejam]

Summary

Legalize G_BITCAST only if both types have a corresponding RegClass.

Authored by: Petar Avramovic

[arsenm]

llvm/lib/Target/AMDGPU/AMDGPULegalizerInfo.cpp

700

You're essentially recreating a less refined version of isRegisterType, but we need the refinements. We can't purely decide this based on the raw bit sizes like this class check does, since we don't want to see something like <64 x i1> as legal. You should either filter out the cases without a corresponding class in isRegisterType or just finally round out the set of classes to cover every possible tuple