[AArch64] Prefer FPRs over GPRs for CLASTB

[AArch64] Prefer FPRs over GPRs for CLASTB

This patch makes the SVE CLASTB GPR alternative more expensive than the
FPR alternative in order to avoid unnecessary cross-file moves.  It also
fixes the prefix used to print the FPR; <vw> only handles 32-bit and
64-bit elements.

2019-08-07  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/aarch64-sve.md (fold_extract_last_<mode>):
	Disparage the GPR alternative relative to the FPR one.
	Fix handling of 8-bit and 16-bit FPR values.

gcc/testsuite/
	* gcc.target/aarch64/sve/clastb_8.c: New test.

From-SVN: r274191