Allocate non-volatile registers in order to be compatible with ABI, regarding gpr_save.
Quoted from https://www.ibm.com/docs/en/ssw_aix_72/assembler/assembler_pdf.pdf page55,
> The preferred method of using GPRs is to use the volatile registers first. Next, use the nonvolatile registers
> in descending order, starting with GPR31.
This patch is based on @jsji 's initial draft.
Tested on test-suite and SPEC, found no degradation.
Reviewed By: jsji, ZarkoCA, xingxue
Differential Revision: https://reviews.llvm.org/D100167
