[PowerPC] Prevent the optimizer from producing wide vector types in IR.

This patch prevents the optimizer from producing wide vectors in the IR,
specifically the MMA types (v256i1, v512i1). The idea is that on Power, we only
want to be producing these types only if the vector_pair and vector_quad types
are used specifically.

To prevent the optimizer from producing these types in the IR,
vectorCostAdjustmentFactor() is updated to return an instruction cost factor or
an invalid instruction cost if the current type is that of an MMA type. An
invalid instruction cost returned by this function signifies to other cost
computing functions to return the maximum instruction cost to inform the
optimizer that producing these types within the IR is expensive, and should not
be produced in the first place.

This issue was first seen in the test case included within this patch.

Differential Revision: https://reviews.llvm.org/D113900

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