Fix some tablegen issues to allow using zero_reg for InstAlias definitions.

Fix some tablegen issues to allow using zero_reg for InstAlias definitions.

This is needed to allow an InstAlias for an instruction with an "OptionalDef"
result register (like ARM's cc_out) where you want to set the optional register
to reg0.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@123490 91177308-0d34-0410-b5e6-96231b3b80d8