cmd/compile: use NOT pseudo-instruction for riscv64 Slicemask

Change-Id: Idefb6669d54929065f57e3bd767c91451dc3a375
Reviewed-on: https://go-review.googlesource.com/c/go/+/223562
Reviewed-by: Cherry Zhang <[email protected]>
Run-TryBot: Cherry Zhang <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

Author: 4a6f656c

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

@@ -143,7 +143,7 @@
 // For positive x, bit 63 of x-1 is always 0, so the result is -1.
 // For zero x, bit 63 of x-1 is 1, so the result is 0.
 //
-(Slicemask <t> x) -> (XORI [-1] (SRAI <t> [63] (ADDI <t> [-1] x)))
+(Slicemask <t> x) -> (NOT (SRAI <t> [63] (ADDI <t> [-1] x)))
 
 // Truncations
 // We ignore the unused high parts of registers, so truncates are just copies.