[Analysis] Ignore casts and unary ops for uninitialized values

A series of unary operators and casts may obscure the variable we're
trying to analyze. Ignore them for the uninitialized value analysis.
Other checks determine if the unary operators result in a valid l-value.

Link: https://github.com/ClangBuiltLinux/linux/issues/1521

Reviewed By: nickdesaulniers

Differential Revision: https://reviews.llvm.org/D114848