Strict implicit conversion rules for Vector types

Previously, the compiler allowed any `__vector` type to be implicitly
convertible with any other `__vector` type.

---
int4 x = 2;
float8 y = x;
short8 z = y / 3;
---

In this release, `__vector` types now, like static arrays, can only be
implicitly converted to either qualified types of itself, or void.

---
const int4 a = 5;
int4 b = a;         // OK, both are int4
void16 c = a;       // OK, cast to void of same size.
float4 d = b;       // Error, cannot implicitly convert int4 to float4.
---

Explicit conversions between `__vector` types can be done, but only between
vectors of the same size.

---
long2 a = 8;
short8 b = cast(short8)a;   // OK
short16 c = cast(short16)b; // Error, cannot cast because of different sizes.
---

Users of the `__simd` and `__simd_sto` intrinsics will have to either add
explicit casts from void16 to the appropriate type at each use, or operate
strictly with void16 vectors until a value is required to be peeked.

---
float4 fun1(float4 a, float4 b)
{
    a = cast(float4)__simd(XMM.ADDPD, a, b);
    a = cast(float4)__simd(XMM.ADDSS, a, b);
    return a;
}

float4 fun2(void16 a, void16 b)
{
    a = __simd(XMM.ADDPD, a, b);
    a = __simd(XMM.ADDSS, a, b);
    return cast(float4)a;
}
---