cmd/compile: convert branch with zero to more optimal branch zero on loong64

This reduces 7500+ instructions from the go toolchain binary on loong64.

	file         before      after      Δ       %
	asm          555066      554406   -660    -0.1189%
	cgo          481480      480764   -716    -0.1487%
	compile      2475836     2474776  -1060   -0.0428%
	cover        516536      515788   -748    -0.1448%
	link         702220      701216   -1004   -0.1430%
	preprofile   238626      238122   -504    -0.2112%
	vet          792798      791894   -904    -0.1140%
	go           1573108     1571676  -1432   -0.0910%
	gofmt        320578      320042   -536    -0.1672%
	total        7656248     7648684  -7564   -0.0988%

Change-Id: I51b70a1543bc258b7664caa8647e75eecbaf5eed
Reviewed-on: https://go-review.googlesource.com/c/go/+/693495
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
Reviewed-by: abner chenc <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: sophie-zhao

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

@@ -951,6 +951,18 @@
 (GEZ (MOVVconst [c]) yes no) && c >= 0 => (First yes no)
 (GEZ (MOVVconst [c]) yes no) && c <  0 => (First no yes)
 
+// Convert branch with zero to more optimal branch zero.
+(BEQ  (MOVVconst [0]) cond yes no) => (EQZ cond yes no)
+(BEQ  cond (MOVVconst [0]) yes no) => (EQZ cond yes no)
+(BNE  (MOVVconst [0]) cond yes no) => (NEZ cond yes no)
+(BNE  cond (MOVVconst [0]) yes no) => (NEZ cond yes no)
+(BLT  (MOVVconst [0]) cond yes no) => (GTZ cond yes no)
+(BLT  cond (MOVVconst [0]) yes no) => (LTZ cond yes no)
+(BLTU (MOVVconst [0]) cond yes no) => (NEZ cond yes no)
+(BGE  (MOVVconst [0]) cond yes no) => (LEZ cond yes no)
+(BGE  cond (MOVVconst [0]) yes no) => (GEZ cond yes no)
+(BGEU (MOVVconst [0]) cond yes no) => (EQZ cond yes no)
+
 // Arch-specific inlining for small or disjoint runtime.memmove
 // Match post-lowering calls, register version.
 (SelectN [0] call:(CALLstatic {sym} dst src (MOVVconst [sz]) mem))