[AArch64] Add MOVPRFX alternatives for SVE EXT patterns

[AArch64] Add MOVPRFX alternatives for SVE EXT patterns

We use EXT both to implement vec_extract for large indices and as a
permute.  In both cases we can use MOVPRFX to handle the case in which
the first input and output can't be tied.

2019-08-15  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/aarch64-sve.md (*vec_extract<mode><Vel>_ext)
	(*aarch64_sve_ext<mode>): Add MOVPRFX alternatives.

gcc/testsuite/
	* gcc.target/aarch64/sve/ext_2.c: Expect a MOVPRFX.
	* gcc.target/aarch64/sve/ext_3.c: New test.

From-SVN: r274515