[Committed] Bug fix to new wi::bitreverse_large function.

[Committed] Bug fix to new wi::bitreverse_large function.

Richard Sandiford was, of course, right to be warry of new code without
much test coverage.  Converting the nvptx backend to use the BITREVERSE
rtx infrastructure, has resulted in far more exhaustive testing and
revealed a subtle bug in the new wi::bitreverse implementation.  The
code needs to use HOST_WIDE_INT_1U (instead of 1) to avoid unintended
sign extension.

This patch has been tested on nvptx-none hosted on x86_64-pc-linux-gnu
(with a minor tweak to use BITREVERSE), where it fixes regressions of
the 32-bit test vectors in gcc.target/nvptx/brev-2.c and the 64-bit
test vectors in gcc.target/nvptx/brevll-2.c.  Committed as obvious.

2023-06-07  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* wide-int.cc (wi::bitreverse_large): Use HOST_WIDE_INT_1U to
	avoid sign extension/undefined behaviour when setting each bit.