rs6000: Convert the vector set variable idx to DImode [PR98914]

rs6000: Convert the vector set variable idx to DImode [PR98914]

vec_insert defines the element argument type to be signed int by ELFv2
ABI.  When expanding a vector with a variable rtx, convert the rtx type
to DImode to support both intrinsic usage and other callers from
rs6000_expand_vector_init produced by v[k] = val when k is long type.

gcc/ChangeLog:

2021-03-21  Xionghu Luo  <luoxhu@linux.ibm.com>

	PR target/98914
	* config/rs6000/rs6000.c (rs6000_expand_vector_set_var_p9):
	Convert idx to DImode.
	(rs6000_expand_vector_set_var_p8): Likewise.

gcc/testsuite/ChangeLog:

2021-03-21  Xionghu Luo  <luoxhu@linux.ibm.com>

	PR target/98914
	* gcc.target/powerpc/pr98914.c: New test.