cmd/compile: small inlining tweak for range-func panics

treat the panic, like a panic.  It helps with inlining,
and thus reduced closure allocation and performance, for
many examples of function range iterators.

Change-Id: Ib1a656cdfa56eb2dee400089c4c94ac14f1d2104
Reviewed-on: https://go-review.googlesource.com/c/go/+/541235
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/compile/internal/inline/inl.go

@@ -528,6 +528,8 @@ opSwitch:
 				case "throw":
 					v.budget -= inlineExtraThrowCost
 					break opSwitch
+				case "panicrangeexit":
+					cheap = true
 				}
 				// Special case for reflect.noescape. It does just type
 				// conversions to appease the escape analysis, and doesn't