JIT: Optimize ConditionalSelect with const zero when condition is not TYP_MASK

[saucecontrol]

AVX-512 embedded masking relies on use of ConditionalSelect, which makes this pattern common:

static Vector512<int> SubtractSaturateUnsigned(Vector512<int> a, Vector512<int> b) =>
    Vector512.ConditionalSelect(Vector512.GreaterThan(a, b), a - b, Vector512<int>.Zero);

       vpcmpgtd k1, zmm0, zmm1
       vpsubd   zmm0 {k1}{z}, zmm0, zmm1

This change improves codegen when the same pattern is used with a compare that does not produce a kmask result:

static Vector256<int> SubtractSaturateUnsigned(Vector256<int> a, Vector256<int> b) =>
    Vector256.ConditionalSelect(Vector256.GreaterThan(a, b), a - b, Vector256<int>.Zero);

        vpcmpgtd ymm2, ymm0, ymm1
        vpsubd   ymm0, ymm0, ymm1
-       vxorps   ymm1, ymm1, ymm1
-       vpblendvb ymm0, ymm1, ymm0, ymm2
+       vpand    ymm0, ymm0, ymm2

And more when pblendvb is not available:

static Vector128<int> SubtractSaturateUnsigned(Vector128<int> a, Vector128<int> b) =>
    Vector128.ConditionalSelect(Vector128.GreaterThan(a, b), a - b, Vector128<int>.Zero);

        movaps   xmm2, xmm0
        pcmpgtd  xmm2, xmm1
        psubd    xmm0, xmm1
        pand     xmm0, xmm2
-       xorps    xmm1, xmm1
-       pandn    xmm2, xmm1
-       por      xmm0, xmm2

Diffs show a few interesting variations in tests.

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[saucecontrol]

cc @tannergooding

[tannergooding]

CC. @dotnet/jit-contrib, @EgorBo for secondary review

[EgorBo]

/azp run Fuzzlyn

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[saucecontrol]

The x86 Fuzzlyn failure looks like another one from #112728. I'll put up a separate fix for that.

Arm failures are #113939