RyuJIT x86: improve localloc codegen in some cases

[BruceForstall]

And reduce #ifdefs.

Address the following TODO in genStackPointerConstantAdjustment():

#ifdef TARGET_X86
    if (regTmp != REG_NA)
    {
        // For x86, some cases don't want to use "sub ESP" because we don't want the emitter to track the adjustment
        // to ESP. So do the work in the count register.
        // TODO-CQ: manipulate ESP directly, to share code, reduce #ifdefs, and improve CQ. This would require
        // creating a way to temporarily turn off the emitter's tracking of ESP, maybe marking instrDescs as "don't
        // track".
        inst_Mov(TYP_I_IMPL, regTmp, REG_SPBASE, /* canSkip */ false);
        inst_RV_IV(INS_sub, regTmp, (target_ssize_t)-spDelta, EA_PTRSIZE);
        inst_Mov(TYP_I_IMPL, REG_SPBASE, regTmp, /* canSkip */ false);
    }
    else
#endif // TARGET_X86

category:cq
theme:stack-allocation
skill-level:expert
cost:small

[TIHan]

This comment and code have been moved to: genStackPointerConstantAdjustment