Produce constructors instead of conversions.

GLSL only supports explicit conversion through constructors. Therefore the conversion
nodes are redundant.

BUG=15415045

Change-Id: Ia6bd93c9c4a69d013a4ec82f81826e6ff5c18eb2
Reviewed-on: https://swiftshader-review.googlesource.com/1113
Reviewed-by: Nicolas Capens <nicolascapens@google.com>