RISCV: clean up target expression handling

The target specific expression handling was slightly regressed by
bbea6425.  This restores the proper
sub-expression evaluation to allow for constant folding within the
expression.  We explicitly discard the layout and assembler when
evaluating the expression to avoid any symbolic computation and instead
using the `evaluateAsRelocatable` to canonicalise and constant fold
only.

We can also simplify the expression handling - none of the target
variants support symbolic difference.  This simplifies the logic for
that and adds additional tests to ensure that we do not accidentally
regress here in the future.

Reviewed By: maskray

Differential Revision: https://reviews.llvm.org/D104473