Omit null initialization of sampler cache pointers
Omit null initialization of sampler cache pointers

The last-used sampler routine can be reused when both the image and
sampler descriptors are the same. To prevent a match on uninitialized
cache keys we previously initialized them both to nullptr (as well as
the function pointer itself), at the entry of the shader routine.
This change reduces it to just initializing the image descriptor. This
suffices since all image instructions use an image descriptor
(but not all use a sampler descriptor).

Bug: b/137649247
Change-Id: I62112f12a58ddefb6e56bddab8e3fdfa76c5b756
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/34628
Tested-by: Nicolas Capens <51edc787c30f24c4904e8ffbb5523c3a9f7a451d@google.com>
Tested-by: Ben Clayton <1f693eb7158dc33015048c09d6712f74dabddf79@google.com>
Kokoro-Presubmit: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Ben Clayton <1f693eb7158dc33015048c09d6712f74dabddf79@google.com>

bkaradzic/SwiftShader,google/swiftshader,google/swiftshader,google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader