cmd/compile: respect Node.Bounded when inserting OpArraySelect

This triggers 119 times during make.bash.

This CL reduces the time it takes for the
compiler to panic while compiling the code in #19751 
from 22 minutes to 15 minutes. Yay, I guess.

Updates #19751 

Change-Id: I8ca7f1ae75f89d1eb2a361d67b3055a975221734
Reviewed-on: https://go-review.googlesource.com/39294
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/gc/ssa.go

@@ -1965,7 +1965,9 @@ func (s *state) expr(n *Node) *ssa.Value {
 					return s.newValue0(ssa.OpUnknown, n.Type)
 				}
 				i = s.extendIndex(i, panicindex)
-				s.boundsCheck(i, s.constInt(Types[TINT], bound))
+				if !n.Bounded() {
+					s.boundsCheck(i, s.constInt(Types[TINT], bound))
+				}
 				return s.newValue1I(ssa.OpArraySelect, n.Type, 0, a)
 			}
 			p := s.addr(n, false)