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Normally 64-bit div/mod is turned into runtime calls on 32-bit
arch, but the front end leaves power-of-two constant division
and hopes the SSA backend turns into a shift or AND. The SSA rule is

(Mod64u <t> n (Const64 [c])) && isPowerOfTwo(c) -> (And64 n (Const64 <t> [c-1]))

But isPowerOfTwo returns true only for positive int64, which leaves
out 1<<63 unhandled. Add a special case for 1<<63.

Fixes #21517.

Change-Id: I02d27dc7177d4af0ee8d7f5533714edecddf8c95
Reviewed-on: https://go-review.googlesource.com/56890

Reviewed-by: Keith Randall <khr@golang.org>