[InstCombine] Remove trivially empty va_start/va_end and va_copy/va_end ranges.

When a va_start or va_copy is immediately followed by a va_end (ignoring
debug information or other start/end in between), then it is safe to
remove the pair. As this code shares some commonalities with the lifetime
markers, this has been factored to helper functions.

This InstCombine pattern kicks-in 3 times when running the LLVM test
suite.

llvm-svn: 269033