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
Fix the build of the gold plugin.

I did the previous patch on OS X and didn't noticed the issue.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221070 91177308-0d34-0410-b5e6-96231b3b80d8
Test 221067 in a fixed-target test so as not to fail on targets with different DWARF encodings

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221071 91177308-0d34-0410-b5e6-96231b3b80d8