cmd/compile: fold negation into subtraction on riscv64

Fold negation into subtraction and avoid double negation.

This removes around 500 instructions from the Go binary on riscv64.

Change-Id: I4aac6c87baa2a0759b180ba87876d488a23df6d7
Reviewed-on: https://go-review.googlesource.com/c/go/+/431105
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Joedian Reid <[email protected]>
Reviewed-by: Wayne Zuo <[email protected]>
Run-TryBot: Joel Sing <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

Author: 4a6f656c

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

@@ -778,6 +778,13 @@
 (SUB  (MOVDconst [0]) x) => (NEG x)
 (SUBW (MOVDconst [0]) x) => (NEGW x)
 
+// Fold negation into subtraction.
+(NEG (SUB x y)) => (SUB y x)
+(NEG <t> s:(ADDI [val] (SUB x y))) && s.Uses == 1 && is32Bit(-val) => (ADDI [-val] (SUB <t> y x))
+
+// Double negation.
+(NEG (NEG x)) => x
+
 // Addition of zero or two constants.
 (ADDI [0] x) => x
 (ADDI [x] (MOVDconst [y])) && is32Bit(x + y) => (MOVDconst [x + y])