cmd/compile: deadcode for LoweredMuluhilo on riscv64

This is a follow up of CL 425101 on RISCV64.

According to RISCV Volume 1, Unprivileged Spec v. 20191213 Chapter 7.1:
If both the high and low bits of the same product are required, then the
recommended code sequence is: MULH[[S]U] rdh, rs1, rs2; MUL rdl, rs1, rs2
(source register specifiers must be in same order and rdh cannot be the
same as rs1 or rs2). Microarchitectures can then fuse these into a single
multiply operation instead of performing two separate multiplies.

So we should not split Muluhilo to separate instructions.

Updates golang#54607

Change-Id: If47461f3aaaf00e27cd583a9990e144fb8bcdb17
Reviewed-on: https://go-review.googlesource.com/c/go/+/425203
Auto-Submit: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Wayne Zuo <wdvxdr@golangcn.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/cmd/compile/internal/ssa/gen/RISCV64.rules

@@ -743,6 +743,10 @@
 (SLTI  [x] (MOVDconst [y])) => (MOVDconst [b2i(int64(y) < int64(x))])
 (SLTIU [x] (MOVDconst [y])) => (MOVDconst [b2i(uint64(y) < uint64(x))])
 
+// deadcode for LoweredMuluhilo
+(Select0 m:(LoweredMuluhilo x y)) && m.Uses == 1 => (MULHU x y)
+(Select1 m:(LoweredMuluhilo x y)) && m.Uses == 1 => (MUL x y)
+
 // Merge negation into fused multiply-add and multiply-subtract.
 //
 // Key:

test/codegen/mathbits.go

@@ -800,12 +800,14 @@ func Mul64(x, y uint64) (hi, lo uint64) {
 
 func Mul64HiOnly(x, y uint64) uint64 {
 	// arm64:"UMULH",-"MUL"
+	// riscv64:"MULHU",-"MUL\t"
 	hi, _ := bits.Mul64(x, y)
 	return hi
 }
 
 func Mul64LoOnly(x, y uint64) uint64 {
 	// arm64:"MUL",-"UMULH"
+	// riscv64:"MUL\t",-"MULHU"
 	_, lo := bits.Mul64(x, y)
 	return lo
 }