[MC] Fix PR45805: infinite recursion in assembler

Give up folding an expression if the fragment of one of the operands
would require laying out a fragment already being laid out. This
prevents hitting an infinite recursion when a fill size expression
refers to a later fragment since computing the offset of that fragment
would require laying out the fill fragment and thus computing its size
expression.

Reviewed By: echristo

Differential Revision: https://reviews.llvm.org/D79570