vect: Add assert to expand_vector_conversion [PR118616]

In some cases (after inliing due to LTO and -O3), GCC cannot
figure out that the length of the converts vect is not empty
when supportable_indirect_convert_operation returns true. So
we get an extra warning because we loop through all but the last
entry and GCC decided that `converts.length () - 1` is -1. This
adds an assert to avoid the warning and maybe even produce slightly
better code for this function. A gcc_checking_assert would be better
here but we don't convert that into an assume attribute or
`if(!a) __builtin_unreachable();`, I filed PR 119439  for that.

Bootstrapped and tested on x86_64-linux-gnu.

	PR tree-optimization/118616
gcc/ChangeLog:

	* tree-vect-generic.cc (expand_vector_conversion): Add
	an assert that converts vect is non empty if
	supportable_indirect_convert_operation returns true.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>

gcc/tree-vect-generic.cc

@@ -1759,6 +1759,7 @@ expand_vector_conversion (gimple_stmt_iterator *gsi)
 					      converts))
     {
       new_rhs = arg;
+      gcc_assert (!converts.is_empty ());
       for (unsigned int i = 0; i < converts.length () - 1; i++)
 	{
 	  new_lhs = make_ssa_name (converts[i].first);