[SimplifyCFG] Make the value type of the hole check bitmask a power-of-2.

When converting a switch to a lookup table we might have to generate a bitmaks
to encode and check for holes in the original switch statement.

The type of this mask depends on the number of switch statements, which can
result in illegal types for pretty much all architectures.

To avoid unnecessary type legalization and help FastISel this commit increases
the size of the bitmask to next power-of-2 value when necessary.

This fixes rdar://problem/18984639.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222168 91177308-0d34-0410-b5e6-96231b3b80d8
Pass a reference to ValueEnumerator.

NFC. This will just make it easier to use std::unique_ptr in a caller.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222170 91177308-0d34-0410-b5e6-96231b3b80d8