Fix bug in implicit cast involving literal float expressions

[vcsharma]

Fixes #2432

[AppVeyorBot]

:white_check_mark: Build DirectXShaderCompiler 1.0.2621 completed (commit https://github.com/Microsoft/DirectXShaderCompiler/commit/30b096b9ce by @vcsharma)

[pow2clk]

From the comments therein, I expect this should also fix #2202

[llvm-beanz]

Since @vcsharma isn't working on DXC anymore I'm going to close this PR. I don't think the approach taken here is the right approach to fix the issue so if we want to fix the underlying issue we will need to go a different direction.

The core issue with this solution is that it "fixes" incorrect codegen by patch fixing the IR during FinishCodeGen. DXC does this for a bunch of things and should not. We should generate the correct cast sequences in the AST, and codegen correctly in the first place.