[AMDGPU][GlobalISel] Revise handling of wide loads in RegBankSelect

When splitting loads in RegBankSelect G_EXTRACT_VECTOR_ELT were being added
which could not be selected. Since invoking the legalizer will generate
instructions that split and combine wide loads, we can remove the redundant
repair instructions which are added by RegBankSelect.

Differential Revision: https://reviews.llvm.org/D75547