Fix remaining build failures with GCC 8.3 (#83266)

When compiling for GCC 8.x (< 8.4), SFINAE is disabled for
iterator_range constructor causing ambiguous resolution to construct an
OperandRange from a MutableOperatorRange, even in the presence of a
static_cast<OperatorRange>. This adds an explicit conversion method to
lift the ambiguity.

Tested with a full MLIR build with GCC 8.3.