[dev.regabi] cmd/compile: fix reporting of overflow

In the previous CL, I had incorrectly removed one of the error
messages from issue20232.go, because I thought go/constant was just
handling it. But actually the compiler was panicking in nodlit,
because it didn't handle constant.Unknown. So this CL makes it leave
n.Type == nil for unknown constant.Values.

While here, also address #42732 by making sure to report an error
message when origConst is called with an unknown constant.Value (as
can happen when multiplying two floating-point constants overflows).

Finally, add OXOR and OBITNOT to the list of operations to report
errors about, since they're also constant expressions that can produce
a constant with a greater bit length than their operands.

Fixes #42732.

Change-Id: I4a538fbae9b3ac4c553d7de5625dc0c87d9acce3
Reviewed-on: https://go-review.googlesource.com/c/go/+/272928
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

test/const2.go

@@ -19,3 +19,14 @@ const LargeB = LargeA * LargeA * LargeA
 const LargeC = LargeB * LargeB * LargeB // GC_ERROR "constant multiplication overflow"
 
 const AlsoLargeA = LargeA << 400 << 400 >> 400 >> 400 // GC_ERROR "constant shift overflow"
+
+// Issue #42732.
+
+const a = 1e+500000000
+const b = a * a // ERROR "constant multiplication overflow"
+const c = b * b
+
+const MaxInt512 = (1<<256 - 1) * (1<<256 + 1)
+const _ = MaxInt512 + 1  // ERROR "constant addition overflow"
+const _ = MaxInt512 ^ -1 // ERROR "constant bitwise XOR overflow"
+const _ = ^MaxInt512     // ERROR "constant bitwise complement overflow"