This PR introduces a step after instruction selection where instructions
can be traversed from the perspective of their validity from the
specification point of view. The PR adds also a way to correct
load/store when there is a type mismatch contradicting the specification
-- an additional bitcast is inserted to keep types consistent.
Correspondent test cases are added and existing test cases are
corrected.
This PR helps to successfully validate with the `spirv-val` tool
(https://github.com/KhronosGroup/SPIRV-Tools) some output that
previously led to validation errors and crashes of back translation from
SPIRV to LLVM IR from the side of SPIRV Translator project
(https://github.com/KhronosGroup/SPIRV-LLVM-Translator).
The added step of bringing instructions to required by the specification
type correspondence can be (should be and will be) extended beyond
load/store instructions to ensure validity rules of other SPIRV
instructions related to type inference.
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