cmd/compile: correct a comment in uint64Tofloat

We only do z>>1 once (as it should be). Remove the extra one
in the comment.

Change-Id: I6fea05fe697de07e160bd686697377babcb3e4c1
Reviewed-on: https://go-review.googlesource.com/c/go/+/347400
Trust: Cherry Mui <[email protected]>
Reviewed-by: David Chase <[email protected]>

Author: cherrymui

src/cmd/compile/internal/ssagen/ssa.go

@@ -5808,7 +5808,6 @@ func (s *state) uint64Tofloat(cvttab *u642fcvtTab, n ir.Node, x *ssa.Value, ft,
 	// } else {
 	// 	  y = uintX(x) ; y = x & 1
 	// 	  z = uintX(x) ; z = z >> 1
-	// 	  z = z >> 1
 	// 	  z = z | y
 	// 	  result = floatY(z)
 	// 	  result = result + result