[mips] Handle MipsMCExpr sub-expression for the MEK_DTPREL tag

[mips] Handle MipsMCExpr sub-expression for the MEK_DTPREL tag

This is a fix for a regression introduced by the rL348194 commit. In
that change new type (MEK_DTPREL) of MipsMCExpr expression was added,
but in some places of the code this type of expression considered as
unexpected.

This change fixes the bug. The MEK_DTPREL type of expression is used for
marking TLS DIEExpr only and contains a regular sub-expression. Where we
need to handle the expression, we retrieve the sub-expression and
handle it in a common way.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@351987 91177308-0d34-0410-b5e6-96231b3b80d8