SpirvShader: Ignore unreachable inputs for Phis.
SpirvShader: Ignore unreachable inputs for Phis.

Bug: b/128527271
Change-Id: I4bf01e7915735d7fd3dc827fd67873fff75a756f
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/28354
Presubmit-Ready: Ben Clayton <1f693eb7158dc33015048c09d6712f74dabddf79@google.com>
Reviewed-by: Nicolas Capens <51edc787c30f24c4904e8ffbb5523c3a9f7a451d@google.com>
Kokoro-Presubmit: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Tested-by: Ben Clayton <1f693eb7158dc33015048c09d6712f74dabddf79@google.com>

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