[CodeGen] Recognize more cases of zero initialization

If a variable has an initializer, codegen tries to build its value. If
the variable is large in size, building its value requires substantial
resources. It causes strange behavior from user viewpoint: compilation
of huge zero initialized arrays like:

    char data_1[2147483648u] = { 0 };

consumes enormous amount of time and memory.

With this change codegen tries to determine if variable initializer is
equivalent to zero initializer. In this case variable value is not
constructed.

This change fixes PR18978.

Differential Revision: https://reviews.llvm.org/D46241

llvm-svn: 332847