cmd/compile: optimize LEAQ address calculations

[rillig]

What version of Go are you using?

go version go1.13.1 windows/amd64

What did you do?

https://play.golang.org/p/Wvp_nntNomk

What did you expect to see?

line 12: LEAQ 0x2(CX)(CX*1), DX

What did you see instead?

line 12: LEAQ 0(CX)(CX*1), DX
line 12: LEAQ 0x2(DX), DX

I'm not sure whether merging these two instructions would make a large difference, I just expect the optimizer to know the instruction set well enough to make this optimization.

[randall77]

This is intentional. See #21735.