We don't natively support BGRA format, instead we handle these
as RGBA and we lower the loads to swap components R and B.

However, the driver emits VPM loads based on the size of the
input variables so when we have a vec2 or float BGRA input,
it would only emit VPM loads for components 0 and 1, which is
not correct since we emit a load of component 2 to swap with
component 0.

v2: handle GL legacy vertex inputs gracefully.

Fixes:
dEQP-VK.draw.output_location.array.b8g8r8a8-unorm-highp-output-vec2
dEQP-VK.draw.output_location.array.b8g8r8a8-unorm-mediump-output-vec2

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Part-of: <mesa/mesa!7271>