[PowerPC] Improve instruction selection bit-permuting operations (64-bit)

This is the second installment of improvements to instruction selection for "bit

permutation" instruction sequences. r224318 added logic for instruction

selection for 32-bit bit permutation sequences, and this adds lowering for

64-bit sequences. The 64-bit sequences are more complicated than the 32-bit

ones because:

a) the 64-bit versions of the 32-bit rotate-and-mask instructions work by replicating the lower 32-bits of the value-to-be-rotated into the upper 32 bits -- and integrating this into the cost modeling for the various bit group operations is non-trivial b) unlike the 32-bit instructions in 32-bit mode, the rotate-and-mask instructions cannot, in one instruction, specify the mask starting index, the mask ending index, and the rotation factor. Also, forming arbitrary 64-bit constants is more complicated than in 32-bit mode because the number of instructions necessary is value dependent.

Plus, support for 'late masking' was added: it is sometimes more efficient to

treat the overall value as if it had no mandatory zero bits when planning the

bit-group insertions, and then mask them in at the very end. Unfortunately, as

the structure of the bit groups is different in the two cases, the more

feasible implementation technique was to generate both instruction sequences,

and then pick the shorter one.

And finally, we now generate reasonable code for i64 bswap:

rldicl 5, 3, 16, 0 rldicl 4, 3, 8, 0 rldicl 6, 3, 24, 0 rldimi 4, 5, 8, 48 rldicl 5, 3, 32, 0 rldimi 4, 6, 16, 40 rldicl 6, 3, 48, 0 rldimi 4, 5, 24, 32 rldicl 5, 3, 56, 0 rldimi 4, 6, 40, 16 rldimi 4, 5, 48, 8 rldimi 4, 3, 56, 0

vs. what we used to produce:

li 4, 255 rldicl 5, 3, 24, 40 rldicl 6, 3, 40, 24 rldicl 7, 3, 56, 8 sldi 8, 3, 8 sldi 10, 3, 24 sldi 12, 3, 40 rldicl 0, 3, 8, 56 sldi 9, 4, 32 sldi 11, 4, 40 sldi 4, 4, 48 andi. 5, 5, 65280 andis. 6, 6, 255 andis. 7, 7, 65280 sldi 3, 3, 56 and 8, 8, 9 and 4, 12, 4 and 9, 10, 11 or 6, 7, 6 or 5, 5, 0 or 3, 3, 4 or 7, 9, 8 or 4, 6, 5 or 3, 3, 7 or 3, 3, 4

which is 12 instructions, instead of 25, and seems optimal (at least in terms

of code size).