Use CONST_VECTOR_ELT instead of XVECEXP

Use CONST_VECTOR_ELT instead of XVECEXP

This patch replaces target-independent uses of XVECEXP with uses
of CONST_VECTOR_ELT.  This kind of replacement isn't necessary
for code specific to targets other than AArch64.

2018-01-02  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* simplify-rtx.c (simplify_const_binary_operation): Use
	CONST_VECTOR_ELT instead of XVECEXP.

From-SVN: r256101