cmd/compile: Fatal instead of panic in large bvbulkalloc

This provides better diagnostics when it occurs.

Updates #19751

Change-Id: I87db54c22e1345891b418c1741dc76ac5fb8ed00
Reviewed-on: https://go-review.googlesource.com/39358
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/gc/bv.go

@@ -29,8 +29,12 @@ type bulkBvec struct {
 
 func bvbulkalloc(nbit int32, count int32) bulkBvec {
 	nword := (nbit + WORDBITS - 1) / WORDBITS
+	size := int64(nword) * int64(count)
+	if int64(int32(size*4)) != size*4 {
+		Fatalf("bvbulkalloc too big: nbit=%d count=%d nword=%d size=%d", nbit, count, nword, size)
+	}
 	return bulkBvec{
-		words: make([]uint32, nword*count),
+		words: make([]uint32, size),
 		nbit:  nbit,
 		nword: nword,
 	}