[PowerPC] Do not increase cost for getUserCost with MMA types

Commit 150681f2 increases
cost of producing MMA types (vector pair and quad).
However, it increases the cost for getUserCost() which is
used in unrolling. As a result, loops that contain these
types already (from the user code) cannot be unrolled
(even with the user's unroll pragma). This was an unintended
sideeffect. Reverting that portion of the commit to allow
unrolling such loops.

Differential revision: https://reviews.llvm.org/D115424