arm: MVE: Fix vec extracts to memory

arm: MVE: Fix vec extracts to memory

This patch fixes vec extracts to memory that can arise from code as seen in the
testcase added. The patch fixes this by allowing mem operands in the set of
mve_vec_extract patterns, which given the only '=r' constraint will lead to the
scalar value being written to a register and then stored in memory using scalar
store pattern.

gcc/ChangeLog:
2020-04-07  Andre Vieira  <andre.simoesdiasvieira@arm.com>

	* config/arm/mve.md (mve_vec_extract*): Allow memory operands in set.

gcc/testsuite/ChangeLog:
2020-04-07  Andre Vieira  <andre.simoesdiasvieira@arm.com>

	* gcc.target/arm/mve/intrinsics/mve_vec_extracts_from_memory.c: New
	test.