Changing the stencil reference shouldn't be causing a new fragment
shader to compile, since it has no effect on the generated program.

This cl removes the front and back stencil reference from the States
structure to avoid uselessly generating programs.

Improves the run times of a few dEQP.KHR_GLES3 tests with SwANGLE:

dEQP.KHR_GLES3/packed_depth_stencil_verify_read_pixels_depth24_stencil8
-> Test runs ~8X faster (from ~4s down to ~500ms)

dEQP.KHR_GLES3/packed_depth_stencil_verify_read_pixels_depth32f_stencil8
-> Test runs ~19X faster (from ~3.8s down to ~200ms)

Bug: b/143288278
Change-Id: Ic46db6822daa924599de0f0a9e9688f8bacc95ea
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/37568
Presubmit-Ready: Alexis Hétu <sugoi@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>