cmd/compile: fold constant in ADDshift op on loong64

Removes 918 instructions from the go binary on loong64.

        file        before      after       Δ
        go          1633120     1632948    -172
        gofmt       323470      323334     -136
        asm         568024      568024     -0
        cgo         488030      487890     -140
        compile     2501050     2500728    -322
        cover       530124      530124     -0
        link        723532      723520     -12
        preprofile  240568      240568     -0
        vet         819392      819256     -136

Change-Id: Id4015c66b2073323b7ad257b3ed05bb99f81e9a1
Reviewed-on: https://go-review.googlesource.com/c/go/+/701655
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Mark Freeman <markfreeman@google.com>

Author: sophie-zhao

src/cmd/compile/internal/ssa/_gen/LOONG64.rules

@@ -730,6 +730,9 @@
 
 (ADDV x0 x1:(SLLVconst [c] y)) && x1.Uses == 1 && c > 0 && c <= 4 => (ADDshiftLLV x0 y [c])
 
+// fold constant in ADDshift op
+(ADDshiftLLV x (MOVVconst [c]) [d]) && is12Bit(c<<d) => (ADDVconst x [c<<d])
+
 // div by constant
 (DIVVU x (MOVVconst [1])) => x
 (DIVVU x (MOVVconst [c])) && isPowerOfTwo(c) => (SRLVconst [log64(c)] x)