Instead of using constructor functions to initialize variables, it is
better to use literal initializer syntax provided by HLSL when it is
possible. This way shader complexity is reduced and constant array
initialization doesn't have to go through as many AST transformations.

Before this patch, vec4 initialization would result in the following
kind of HLSL:

float4 f = float4(1.0, 2.0, 3.0, 4.0);

After this patch, it will be:

float4 f = {1.0, 2.0, 3.0, 4.0};

Before this patch, vec2 array initialization would result in the
following kind of HLSL:

float2 f[2] = {0, 0, 0, 0};
angle_construct_into_2_float2(f, float2(1.0, 2.0), float2(3.0, 4.0));

After this patch, it will be:

float2 f[2] = {1.0, 2.0, 3.0, 4.0};

BUG=angleproject:1094
BUG=541551
TEST=WebGL conformance tests

Change-Id: I9816a8d95a2cba3964922f6b561862d478da6145
Reviewed-on: https://chromium-review.googlesource.com/311160Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>