IR: MDNode => Value: Add Instruction::getMDNode()

Add `Instruction::getMDNode()` that casts to `MDNode` before changing
`Instruction::getMetadata()` to return `Value`.  This avoids adding
`cast_or_null<MDNode>` boiler-plate throughout the code.

Part of PR21433.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221023 91177308-0d34-0410-b5e6-96231b3b80d8
IR: MDNode => Value: Instruction::getMetadata()

Change `Instruction::getMetadata()` to return `Value` as part of
PR21433.

Update most callers to use `Instruction::getMDNode()`, which wraps the
result in a `cast_or_null<MDNode>`.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221024 91177308-0d34-0410-b5e6-96231b3b80d8
IR: MDNode => Value: Instruction::getAllMetadata()

Change `Instruction::getAllMetadata()` to modify a vector of `Value`
instead of `MDNode` and update call sites.  This is part of PR21433.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221027 91177308-0d34-0410-b5e6-96231b3b80d8
Temporarily revert r220777 to sort out build bot breakage.
"[x86] Simplify vector selection if condition value type matches vselect value type and true value is all ones or false value is all zeros."

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221028 91177308-0d34-0410-b5e6-96231b3b80d8