cmd/compile: avoid extending when already sufficiently masked on loong64

Reduce the number of go toolchain instructions on loong64 as follows.

        file         before      after      Δ       %
        asm          562400      561348   -1052   -0.1871%
        cgo          488268      487932   -336    -0.0688%
        compile      2505750     2503710  -2040   -0.0814%
        cover        525874      525506   -368    -0.0700%
        link         714162      713562   -600    -0.0840%
        preprofile   240250      240054   -196    -0.0816%
        vet          794074      793742   -332    -0.0418%
        go           1598284     1597680  -604    -0.0378%
        gofmt        324126      323926   -200    -0.0617%
        total        7753188     7747460  -5728   -0.0739%

Change-Id: I3406113553b0fc4947d763ab66e57e69aa98c020
Reviewed-on: https://go-review.googlesource.com/c/go/+/683615
Reviewed-by: Keith Randall <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Mark Freeman <[email protected]>

Author: sophie-zhao

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

@@ -843,6 +843,14 @@
 
 (MOVBUreg (ANDconst [c] x)) => (ANDconst [c&0xff] x)
 
+// Avoid extending when already sufficiently masked.
+(MOVBreg  x:(ANDconst [c] y)) && c >= 0 && int64(int8(c)) == c => x
+(MOVHreg  x:(ANDconst [c] y)) && c >= 0 && int64(int16(c)) == c => x
+(MOVWreg  x:(ANDconst [c] y)) && c >= 0 && int64(int32(c)) == c => x
+(MOVBUreg x:(ANDconst [c] y)) && c >= 0 && int64(uint8(c)) == c => x
+(MOVHUreg x:(ANDconst [c] y)) && c >= 0 && int64(uint16(c)) == c => x
+(MOVWUreg x:(ANDconst [c] y)) && c >= 0 && int64(uint32(c)) == c => x
+
 // Prefetch instructions (hint specified using aux field)
 // For PRELD{,X} A value of hint indicates:
 //    hint=0 is defined as load prefetch to L1-cache