[AMDGPU][GlobalISel] Check exact width in get*ClassForBitWidth and widen if necessary

Instead of checking if the given bitwidth is less or equal to a bitwidth of an existing RegClass,
check if it has the exact same value.

For LLVM vector types that don't have a corresponding Register Class, widen them during legalization.
That goes for G_EXTRACT_VECTOR_ELT, G_INSERT_VECTOR_ELT and G_BUILD_VECTOR.

Differential revision: https://reviews.llvm.org/D148096
Reviewers: foad, arsenm