[TableGen] Fix spurious type error in bit assignment.

Summary:
The following example gives the error message "expected value of type
'bits<32>', got 'bit'" on the assignment.

    class Instruction { bits<32> encoding; }
    def foo: Instruction { let encoding{10} = !eq(0, 1); }

But there's nothing wrong with this code: 'bit' is a perfectly good
type for the RHS of an assignment to a //single bit// of an
instruction encoding.

The problem is that `ParseBodyItem` is accidentally type-checking the
RHS against the full type of the `encoding` field, without adjusting
it in the case where we're only assigning to a subset of the bits. The
fix is trivial.

Reviewers: nhaehnle, hfinkel

Reviewed By: hfinkel

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74220