GLSL supports constructing vectors from matrices. In the case
of vec3(mat2), vec4(mat2) or vec4(mat3), a second instruction
is required to copy the remaining matrix elements from the
2nd row of the matrix into the rest of the vector.

Fixes 6 webgl conformance tests:
conformance/glsl/constructors/glsl-construct-vec3.html
conformance/glsl/constructors/glsl-construct-vec4.html
conformance/glsl/constructors/glsl-construct-ivec3.html
conformance/glsl/constructors/glsl-construct-ivec4.html
conformance/glsl/constructors/glsl-construct-bvec3.html
conformance/glsl/constructors/glsl-construct-bvec4.html

Change-Id: I85ffd3b60623d0dce2bd7c0027748e1829a27dbc
Reviewed-on: https://swiftshader-review.googlesource.com/16469Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>