Remove unnecessary null pointer checks

[elfring]

An extra null pointer check is not needed in functions like the following.

• ShaderMerger::BuildEsGsMergedShader
• ShaderMerger::BuildLsHsMergedShader

[linqun]

I agree, it is not necessary. we will review whole llpc folder and fix it step by step after Chinese New Year.
of course, I will be appreciate if you can help to fix it :)

[JacobHeAMD]

Will not fix, keep them for safe

[elfring]

How do you think about to use a development tool like “clang-tidy” for corresponding source code adjustments?