[AMDGPU] Use SGPR instead of SReg classes

12994a70 did this for 128-bit classes:

    SGPR_128 only includes the real allocatable SGPRs, and SReg_128 adds
    the additional non-allocatable TTMP registers. There's no point in
    allocating SReg_128 vregs. This shrinks the size of the classes
    regalloc needs to consider, which is usually good.

This patch extends it to all classes > 64 bits, for consistency.

Differential Revision: https://reviews.llvm.org/D78622