nvptx: Provide vec_set<mode> and vec_extract<vmode><mode> patterns

nvptx: Provide vec_set<mode> and vec_extract<vmode><mode> patterns

This patch provides standard vec_extract and vec_set patterns to the
nvptx backend, to extract an element from a PTX vector and set an
element of a PTX vector respectively.  PTX vectors (I hesitate to
call them SIMD vectors) may contain up to four elements, so vector
modes up to size four are supported by this patch even though the
nvptx backend currently only allows V2SI and V2DI, i.e. two out
of the ten possible vector modes.

As an example of the improvement, the following C function:

typedef int __v2si __attribute__((__vector_size__(8)));
int foo (__v2si arg) { return arg[0]+arg[1]; }

previously generated this code using a shift:

  mov.u64     %r25, %ar0;
  ld.v2.u32   %r26, [%r25];
  mov.b64     %r28, %r26;
  shr.s64     %r30, %r28, 32;
  cvt.u32.u32 %r31, %r26.x;
  cvt.u32.u64 %r32, %r30;
  add.u32     %value, %r31, %r32;

but with this patch now generates:

  mov.u64   %r25, %ar0;
  ld.v2.u32 %r26, [%r25];
  mov.u32   %r28, %r26.x;
  mov.u32   %r29, %r26.y;
  add.u32   %value, %r28, %r29;

I've implemented these getters and setters as their own instructions
instead of attempting the much more intrusive patch of changing the
backend's definition of register_operand.  Given the limited utility
of PTX vectors, I'm not convinced that attempting to support them as
operands in every instruction would be worth the effort involved.

This patch has been tested on nvptx-none hosted on x86_64-pc-linux-gnu
with "make" and "make check" with no new regressions.

2020-07-15  Roger Sayle  <roger@nextmovesoftware.com>
	    Tom de Vries  <tdevries@suse.de>

gcc/ChangeLog:

	* config/nvptx/nvptx.md (nvptx_vector_index_operand): New predicate.
	(VECELEM): New mode attribute for a vector's uppercase element mode.
	(Vecelem): New mode attribute for a vector's lowercase element mode.
	(*vec_set<mode>_0, *vec_set<mode>_1, *vec_set<mode>_2)
	(*vec_set<mode>_3): New instructions.
	(vec_set<mode>): New expander to generate one of the above insns.
	(vec_extract<mode><Vecelem>): New instruction.

gcc/testsuite/ChangeLog:

	* gcc.target/nvptx/v2si-vec-set-extract.c: New test.