[AArch64] Don't apply mode_for_int_vector to scalars

[AArch64] Don't apply mode_for_int_vector to scalars

aarch64_emit_approx_sqrt handles both vectors and scalars and was using
mode_for_int_vector even for the scalar case.  Although that happened
to work, it isn't how mode_for_int_vector is supposed to be used.

2019-10-23  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/aarch64.c (aarch64_emit_approx_sqrt): Use
	int_mode_for_mode rather than mode_for_int_vector for scalars.

From-SVN: r277311