tree-optimization/123755 - fix LEN-masking of trapping calls

tree-optimization/123755 - fix LEN-masking of trapping calls

There's multiple issues with properly handling len-masking of calls
that might trap.  Similar to get_conditional_internal_fn,
get_len_internal_fn expects a COND_* argument only.  When the
original call is not already masked computation and code-gen fails
to add mask and else arguments.

This fixes gcc.target/riscv/rvv/autovec/reduc/reduc_call-5.c

	PR tree-optimization/123755
	* tree-vect-stmts.cc (vectorizable_call): Fixup LEN masking
	of unconditional but possibly trapping calls.