Update nvptx's bitrev<mode>2 pattern to use BITREVERSE rtx.

Update nvptx's bitrev<mode>2 pattern to use BITREVERSE rtx.

This minor tweak to the nvptx backend switches the representation of
of the brev instruction from an UNSPEC to instead use the new BITREVERSE
rtx.  This allows various RTL optimizations including evaluation (constant
folding) of integer constant arguments at compile-time.

	gcc/
	* config/nvptx/nvptx.md (UNSPEC_BITREV): Delete.
	(bitrev<mode>2): Represent using bitreverse.
	gcc/testsuite/
	* gcc.target/nvptx/brev-2-O2.c: Adjust.
	* gcc.target/nvptx/brevll-2-O2.c: Likewise.

Co-authored-by: Thomas Schwinge <thomas@codesourcery.com>