JitArm64: Optimize multiplication

[Sintendo]

Optimize multiplication for various constants. We introduce a MultiplyImmediate function, which contains the logic much like the one that exists for x86, and reuse it in both mulli and mullwx.

Also a minor register allocation improvement for mulli.

---

Multiplication by 0

Before:

0x52800019   mov    w25, #0x0
0x1b197f5b   mul    w27, w26, w25

After:

Multiplication by 1 (example 1)

Before:

0x52800038   mov    w24, #0x1
0x1b1a7f1b   mul    w27, w24, w26

After:

0x2a1a03fb   mov    w27, w26

Multiplication by 1 (example 2)

Before:

0x52800039   mov    w25, #0x1
0x1b1a7f3a   mul    w26, w25, w26

After:

Multiplication by -1

Before:

0x12800015   mov    w21, #-0x1
0x1b157f7b   mul    w27, w27, w21

After:

0x4b1b03fb   neg    w27, w27

Multiplication by 2^n

Before:

0x52800817   mov    w23, #0x40
0x1b167ef6   mul    w22, w23, w22

After:

0x531a66d6   lsl    w22, w22, #6

Multiplication by 2^n + 1

Before:

0x52800838   mov    w24, #0x41
0x1b187f7b   mul    w27, w27, w24

After:

0x0b1b1b7b   add    w27, w27, w27, lsl #6

mulli register allocation

Before:

0x52800659   mov    w25, #0x32
0x1b197f5b   mul    w27, w26, w25

After:

0x5280065b   mov    w27, #0x32
0x1b1b7f5b   mul    w27, w26, w27

[Sintendo]

Added two more interesting cases. The -(2^n) case also covers -1, so explicit handling of that has been dropped.

---

Multiplication by -(2^n)

Before:

0x128001f7   mov    w23, #-0x10
0x1b1a7efa   mul    w26, w23, w26
0x93407f58   sxtw   x24, w26

After:

0x4b1a13fa   neg    w26, w26, lsl #4
0x93407f58   sxtw   x24, w26

Multiplication by -(2^n) + 1

Before:

0x1280005b   mov    w27, #-0x3
0x1b1b7f18   mul    w24, w24, w27

After:

0x4b180b18   sub    w24, w24, w24, lsl #2

[Rumi-Larry]

How about the multiplication by -1 case?

[Sintendo]

It's handled by the -(2^n) case now, no need to handle it separately.

[AdmiralCurtiss]

@JosJuice Can you re-check this?

[JosJuice]

Still LGTM. But would I prefer to wait with merging this kind of change since we're so close to a beta.

[AdmiralCurtiss]

Well, since we pushed the beta...