[X86] Factor out new helper getPSHUFB

I found three implementations of this.  This splits it out into a new function
and uses it from the three places.

My plan is to add a fourth use when lowering a vector_shuffle:v16i16.

Compared the assembly output of test/CodeGen/X86 before and after.

The only change is due to how the first PSHUFB was generated in
LowerVECTOR_SHUFFLEv8i16.  If the shuffle mask specified undef (i.e. -1), the
old implementation would write -1 * 2 and -1 * 2 + 1 (254 and 255) in the
control mask.  Now we write 0x80.  These are of course interchangeable since
bit 7 decides if a constant zero is written in the result byte.  The other
instances of this code use 0x80 consistently.

Related to <rdar://problem/16167303>
[X86] Generate VPSHUFB for in-place v16i16 shuffles

This used to resort to splitting the 256-bit operation into two 128-bit
shuffles and then recombining the results.

Fixes <rdar://problem/16167303>