OpImageSampleExplicitLod can either have a Lod or Grad operand.

Bug: b/129523279
Test: dEQP-VK.glsl.texture_functions.texturegrad.sampler2d_fixed_fragment
Change-Id: I0c4a885cc6833614572ed7a061b53c9f41838f0b
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/30032
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>