Use zero register in csel when possible

[SwapnilGaikwad]

For

static void foo(int a, int b) {
    if(a == 5) {
        b = 0;
    }
    consume<int>(a, b);
}

Without this patch we get

...
2A1F03E2          mov     w2, wzr     <=== avoidable mov
7100141F          cmp     w0, #5
1A810041          csel    w1, w2, w1, eq
D29EA202          movz    x2, #0xF510
F2A85BC2          movk    x2, #0x42DE LSL #16
F2DFFFE2          movk    x2, #0xFFFF LSL #32
F9400042          ldr     x2, [x2]
D63F0040          blr     x2
...

After:

...
7100141F          cmp     w0, #5
1A8103E1          csel    w1, wzr, w1, eq
D29EA202          movz    x2, #0xF510
F2AF0D62          movk    x2, #0x786B LSL #16
F2DFFFE2          movk    x2, #0xFFFF LSL #32
F9400042          ldr     x2, [x2]
D63F0040          blr     x2
...

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Issue Details

---

For

static void foo(int a, int b) {
    if(a == 5) {
        b = 0;
    }
    consume<int>(a, b);
}

Without this patch we get

...
2A1F03E2          mov     w2, wzr     <=== avoidable mov
7100141F          cmp     w0, #5
1A810041          csel    w1, w2, w1, eq
D29EA202          movz    x2, #0xF510
F2A85BC2          movk    x2, #0x42DE LSL #16
F2DFFFE2          movk    x2, #0xFFFF LSL #32
F9400042          ldr     x2, [x2]
D63F0040          blr     x2
...

After:

...
7100141F          cmp     w0, #5
1A8103E1          csel    w1, wzr, w1, eq
D29EA202          movz    x2, #0xF510
F2AF0D62          movk    x2, #0x786B LSL #16
F2DFFFE2          movk    x2, #0xFFFF LSL #32
F9400042          ldr     x2, [x2]
D63F0040          blr     x2
...

Author:	SwapnilGaikwad
Assignees:	-
Labels:	area-CodeGen-coreclr
Milestone:	-