https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70501

This fixes 70501.  The cause is an omission in typeck when converting a scalar 
operand to a vector.  We use build_vector_from_val, which can return a 
CONSTRUCTOR.  We fail to wrap that CONSTRUCTOR in a TARGET_EXPR.

The ICE arises because at the point we meet that CONSTRUCTOR during the 
constexpr processing, the currently active object under construction is that for 
the result of the <= operator, which has type vector-of-bool, rather than 
vector-of-int. (thus this  problem arises in other vector  ops, but mostly 
undetected because the result type is the same  as the operand type)

ok?

nathan