instcombine: merge the functions that remove dead allocas and dead mallocs/callocs/...

This patch removes ~70 lines in InstCombineLoadStoreAlloca.cpp and makes both functions a bit more aggressive than before :)
In theory, we can be more aggressive when removing an alloca than a malloc, because an alloca pointer should never escape, but we are not taking advantage of this anyway

llvm-svn: 159952