There is a subtle trap here -- almost all SPIRV instructions require vector
widths to match, but for OpVectorShuffle, the widths of the result
vector and the two sources are completely independent.

Fixes dEQP-VK.glsl.operator.unary_operator.plus.highp_vec2_vertex and many others.

Bug: b/127959969
Change-Id: Iaa2cc09fb510fa2ab07a6f53599af8dade553a9a
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/26570
Presubmit-Ready: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>