Revert "[MISched] Introduce and use ResourceSegments."

Reverted because it produces the following builbot failure at https://lab.llvm.org/buildbot#builders/9/builds/27319:

/b/ml-opt-rel-x86-64-b1/llvm-project/llvm/unittests/CodeGen/SchedBoundary.cpp: In member function ‘virtual void ResourceSegments_getFirstAvailableAtFromBottom_empty_Test::TestBody()’:
/b/ml-opt-rel-x86-64-b1/llvm-project/llvm/unittests/CodeGen/SchedBoundary.cpp:395:31: error: call of overloaded ‘ResourceSegments(<brace-enclosed initializer list>)’ is ambiguous
 395 |   auto X = ResourceSegments({});
     |                               ^

This reverts commit dc312f03.