[PowerPC] Collapse RLDICL/RLDICR into RLDIC when possible

Generally speaking, we lower to an optimal rotate sequence for nodes visible in
the SDAG. However, there are instances where the two rotates are not visible at
ISEL time - most notably those in a very common sequence when lowering switch
statements to jump tables.

A common situation is a switch on a 32-bit integer. This value has to have the
upper 32 bits cleared and because jump table offsets are word offsets, the value
needs to be shifted left by 2 bits. We currently emit the clear and the left
shift as two separate instructions, but this is not needed as we can lower it to
a single RLDIC.

This patch just cleans that up.

Differential revision: https://reviews.llvm.org/D60402

llvm-svn: 362576