[rs6000] Use VIEW_CONVERT_EXPR to reinterpret vectors (PR 92515)

[rs6000] Use VIEW_CONVERT_EXPR to reinterpret vectors (PR 92515)

The new tree-cfg.c checking in r278245 tripped on folds of
ALTIVEC_BUILTIN_VPERM_*, which were using gimple_convert
rather than VIEW_CONVERT_EXPR to reinterpret the contents
of a vector as a different type.

2019-11-15  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR target/92515
	* config/rs6000/rs6000-call.c (rs6000_gimple_fold_builtin): Use
	VIEW_CONVERT_EXPR to reinterpret vectors as different types.

From-SVN: r278292