[DirectX] Support typedBufferLoad and Store for `RWBuffer<double2>`

[bogner]

HLSL's Buffer/RWBuffer can operate on a vector of two doubles, like so: https://hlsl.godbolt.org/z/8sa3v8zMn

The typedBufferLoad and Store dxil intrinsics (introduced in #104252 and #104253) should be taught to handle a vector of 2 doubles and lower to BufferLoad and Store ops with the appropriate splitDouble and makeDouble ops as necessary.