[AMDGPU] Clean up SReg classes

Remove unused LO16 classes SReg_LO16_XM0_XEXEC, SReg_LO16_XEXEC_HI and
SReg_LO16_XM0.

Simplify the definition of SReg_32.

Add SReg_32_XEXEC and use it to improve SReg_1_XEXEC which previously
excluded M0 for no good reason.

Improve SReg_1 which previously excluded EXEC_HI for no good reason.

Differential Revision: https://reviews.llvm.org/D140012