[IR] Don't call assertModuleIsMaterialized in release builds

Summary:
To fix a release vs debug build linking error, r259695 made the body of assertModuleIsMaterialized empty if Value.cpp gets compiled in a release build. This way any code compiled as a debug build can still link against a release version of the function.

This patch takes this a step farther and removes all calls to it from Value.h in any code that includes it in a relase build.

This shrinks the opt binary on my macbook build by 17240 bytes.

Reviewers: rafael

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D28191

llvm-svn: 291883