[flang] Minor fix in folding of reshape intrinsic (flang-compiler/f18#842)

The standard permits source to have more elements than the
result (as specified by the SHAPE arg). While copying, ensure
that we do not copy more than the number of elements of the
result.

Original-commit: flang-compiler/f18@6b8284f4f2cff32fcf5483ea93d1d239ecdacd33
Reviewed-on: https://github.com/flang-compiler/f18/pull/842