cmd/compile: int64(uint64 >> x) >= 0 if x > 0

This rewrite rule triggers only once, in math/big.quotToFloat64,
as part of converting a uint64 to a float64.

Nevertheless, it is cheap; let's add it.

Change-Id: I3ed4a197a559110fec1bc04b3a8abb4c7fcc2c89
Reviewed-on: https://go-review.googlesource.com/c/go/+/167500
Run-TryBot: Josh Bleecher Snyder <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Keith Randall <[email protected]>

Author: josharian

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

@@ -425,6 +425,8 @@
 (Geq32 (And32 _ (Const32 [c])) (Const32 [0])) && int32(c) >= 0 -> (ConstBool [1])
 (Geq64 (And64 _ (Const64 [c])) (Const64 [0])) && int64(c) >= 0 -> (ConstBool [1])
 
+(Geq64 (Rsh64Ux64 _ (Const64 [c])) (Const64 [0])) && c > 0 -> (ConstBool [1])
+
 (Greater64U (Const64 [c]) (Const64 [d])) -> (ConstBool [b2i(uint64(c) > uint64(d))])
 (Greater32U (Const32 [c]) (Const32 [d])) -> (ConstBool [b2i(uint32(c) > uint32(d))])
 (Greater16U (Const16 [c]) (Const16 [d])) -> (ConstBool [b2i(uint16(c) > uint16(d))])