glsl: Fix conversions in array constructors

glsl: Fix conversions in array constructors

Array constructors obey narrower conversion rules than other constructors
[1] --- they use the implicit conversion rules [2] instead of the scalar
constructor conversions [3].  But process_array_constructor() was
incorrectly applying the broader rules.

[1] GLSL 1.50 spec, Section 5.4.4 Array Constructors, page 52 (58 of pdf)
[2] GLSL 1.50 spec, Section 4.1.10 Implicit Conversions, page 25 (31 of pdf)
[3] GLSL 1.50 spec, Section 5.4.1 Conversion, page 48 (54 of pdf)

To fix this, first check (with glsl_type::can_be_implicitly_converted_to)
if an implicit conversion is legal before performing the conversion.

Fixes:
piglit:spec/glsl-1.20/compiler/structure-and-array-operations/array-ctor-implicit-conversion-bool-float.vert
piglit:spec/glsl-1.20/compiler/structure-and-array-operations/array-ctor-implicit-conversion-bvec*-vec*.vert

Note: This is a candidate for the 7.10 and 7.11 branches.
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Chad Versace <chad@chad-versace.us>