SwiftShader already extends unsigned texels with 0s and signed texels
with the sign bit. So all that's needed is for all operations to accept
the operands.

Unless a stakeholder specifies that dEQP is undertesting these
operands. This is tracked by b/174475384.

Tests: dEQP-VK.image.extend_operands_spirv1p4.*
Bug: b/173046235
Change-Id: Ib7024b533cefd52957bc7275a041e06ec179c829
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/50768Tested-by: Sean Risser <srisser@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>