nvptx: Support 16-bit shifts and extendqihi2

nvptx: Support 16-bit shifts and extendqihi2

Add support for 16-bits shifts and for sign extension from 8 bits to
16 bits.

This patch has been tested on nvptx-none with no new regressions.

2020-07-28  Roger Sayle  <roger@nextmovesoftware.com>
	    Tom de Vries  <tdevries@suse.de>

gcc/ChangeLog:

	* config/nvptx/nvptx.md (extendqihi2): New instruction.
	(ashl<mode>3, ashr<mode>3, lshr<mode>3): Support HImode.

gcc/testsuite/ChangeLog:

	* gcc.target/nvptx/cvt.c: New test.
	* gcc.target/nvptx/shift16.c: New test.