Remove DwarfUnit::LabelEnd in favor of computing the length of the section directly

This was a compile-unit specific label (unused in type units) and seems
unnecessary anyway when we can more easily directly compute the size of
the compile unit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221067 91177308-0d34-0410-b5e6-96231b3b80d8
Remove test coverage added in 221067 due to it being non-portable.

Will try to find a portable way to test this (or a fixed-target test I
can add such coverage to) shortly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221068 91177308-0d34-0410-b5e6-96231b3b80d8
InstCombine: Don't assume that m_ZExt matches an Instruction

m_ZExt might bind against a ConstantExpr instead of an Instruction.
Assuming this, using cast<Instruction>, results in InstCombine crashing.

Instead, introduce ZExtOperator to bridge both Instruction and
ConstantExpr ZExts.

This fixes PR21445.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221069 91177308-0d34-0410-b5e6-96231b3b80d8