[LiveDebugValues] Allow EntryValue with OP_deref expressions

With D68945, more DBG_VALUEs were created without the indirect operand,
instead relying on OP_deref to accomplish the same effect.

At the time, however, we were not able to handle arbitrary expressions
in combination with OP_LLVM_entry_value, so D71416 prevented the use of
such operation in the presence of expressions.

As per the comment in DIExpression::isValid, "we support only entry
values of a simple register location." As such, a simple deref operation
should be supported. In fact, D80345 added support for indirect
DBG_VALUEs.

Taken the patches above into consideration, this commit relaxes the
restrictions on which expressions are allowed for entry value
candidates: the expression must be either empty or a single dereference
operator.

This patch is useful for D141381, which adds support for storing the
address of ABI-indirect parameters on the stack.

Depends on D142160

Differential Revision: https://reviews.llvm.org/D142654