Since EOpMatrixSwizzle is a new op, existing back-ends only work when the
front end first decomposes it to other operations. So far, this is only
being done for simple assignment into matrix swizzles.
This partially addressess issue #670, for when the matrix swizzle
degenerates to a component or column: m[c], m[c][r] (where HLSL
swaps rows and columns for user's view).
An error message is given for the arbitrary cases not covered.
These cases will work for arbitrary use of l-values.
Future work will handle more arbitrary swizzles, which might
not work as arbitrary l-values.
