fix validation of dual source blending structs

[vxpm]

Description
validation for structs that contain a blend_src attribute in one of their members is wrong. the WebGPU spec says:

For each structure type S defined in a WGSL module (not just those used in shader stage inputs or outputs), let members be the set of members of S that have location attributes.

If any entry in members specifies a blend_src attribute, members must use dual source blending, which means:

• members must contain exactly 2 entries, one with @location(0) @blend_src(0) and one with @location(0) @blend_src(1).
• All the members must have same data type.

Otherwise, members must not contain two entries with the same location value.

(source)

the part where naga is wrong is this one:

let members be the set of members of S that have location attributes

currently, naga uses all the members of S during validation, instead of just the members with location attributes. this means you can't use any members with builtin attributes (such as frag_depth).

this PR fixes this by only checking members with location attributes.

related PR: #8856