Correct gcc vector splat conversion from float to int-vector

In looking into some other code, I came across this issue where a
float converted to a gcc integer vector via a splat causes it to miss
the float-to-integral cast, which causes some REALLY strange codegen
bugs.

The AST looked like:
`-ImplicitCastExpr <col:13>
'gcc_int_2':'__attribute__((__vector_size__(2 * sizeof(int)))) int' <VectorSplat>
        `-ImplicitCastExpr <col:13> 'float' <LValueToRValue>
                  `-DeclRefExpr <col:13> 'float' lvalue ParmVar
                  0x556f16a5dc90 'f' 'float'

Despite the type of the VectorSplat cast as printed, it ended up
becoming a vector of float, which caused non-matching instructions. For
example, IntVector + a float constant resulted in:

add <2 x i32> %8, <2 x float> <float 3.000000e+00, float 3.000000e+00>

This patch corrects the conversion so that the float is first converted
to an integral, THEN splatted.