[HLSL] implement `lerp` intrinsic

[llvm-beanz]

Implement HLSL lerp intrinsic:

https://learn.microsoft.com/en-us/windows/win32/direct3dhlsl/dx-graphics-hlsl-lerp

[farzonl]

The remaining work here is elementwise lerps.

Unlike other elementwise operations lerp tends to do an odd grouping.

for example:
https://godbolt.org/z/ses881sx7

export float3 dot1(float3 x, float3 y, float3 z) {
    return lerp(x, y, z);
}

This groups the z vector and extracts all its elements first

  %z.i0 = extractelement <3 x float> %z, i32 0, !dbg !41 ; line:3 col:12
  %z.i1 = extractelement <3 x float> %z, i32 1, !dbg !41 ; line:3 col:12
  %z.i2 = extractelement <3 x float> %z, i32 2, !dbg !41 ; line:3 col:12
  %x.i0 = extractelement <3 x float> %x, i32 0, !dbg !41 ; line:3 col:12
  %y.i0 = extractelement <3 x float> %y, i32 0, !dbg !41 ; line:3 col:12

It also groups the multiplies and adds

  %.i01 = fmul fast float %z.i0, %.i0, !dbg !41 ; line:3 col:12
  %.i12 = fmul fast float %z.i1, %.i1, !dbg !41 ; line:3 col:12
  %.i23 = fmul fast float %z.i2, %.i2, !dbg !41 ; line:3 col:12
  %.i04 = fadd fast float %.i01, %x.i0, !dbg !41 ; line:3 col:12
  %.i15 = fadd fast float %.i12, %x.i1, !dbg !41 ; line:3 col:12
  %.i26 = fadd fast float %.i23, %x.i2, !dbg !41 ; line:3 col:12

[llvm-beanz]

We do not need clang to match the instruction ordering emitted by DXC.