Improve code generation for `toCtorIdx`

It should be possible to replace

@[extern c inline "(uint64_t)#1"]
def BinderInfo.toUInt64 : BinderInfo → UInt64
  | .default        => 0
  | .implicit       => 1
  | .strictImplicit => 2
  | .instImplicit   => 3

with

def BinderInfo.toUInt64 (bi : BinderInfo) : UInt64 :=
  bi.toCtorIdx.toUInt64

without pessimizing the generated code. However, this is not currently the case as toCtorIdx is compiled as a regular match and clang doesn't know enough about the domain of BinderInfo to get rid of those branches.

0000000001e0ec30 <l_Lean_BinderInfo_toUInt64>:
 1e0ec30:	85 ff                	test   %edi,%edi
 1e0ec32:	74 15                	je     1e0ec49 <l_Lean_BinderInfo_toUInt64+0x19>
 1e0ec34:	40 80 ff 02          	cmp    $0x2,%dil
 1e0ec38:	74 18                	je     1e0ec52 <l_Lean_BinderInfo_toUInt64+0x22>
 1e0ec3a:	40 80 ff 01          	cmp    $0x1,%dil
 1e0ec3e:	75 1b                	jne    1e0ec5b <l_Lean_BinderInfo_toUInt64+0x2b>
 1e0ec40:	b8 03 00 00 00       	mov    $0x3,%eax
 1e0ec45:	48 d1 e8             	shr    %rax
 1e0ec48:	c3                   	ret
 1e0ec49:	b8 01 00 00 00       	mov    $0x1,%eax
 1e0ec4e:	48 d1 e8             	shr    %rax
 1e0ec51:	c3                   	ret
 1e0ec52:	b8 05 00 00 00       	mov    $0x5,%eax
 1e0ec57:	48 d1 e8             	shr    %rax
 1e0ec5a:	c3                   	ret
 1e0ec5b:	b8 07 00 00 00       	mov    $0x7,%eax
 1e0ec60:	48 d1 e8             	shr    %rax
 1e0ec63:	c3                   	ret