[x86] Update the exact FileCheck syntax of the 256-bit and 512-bit

[x86] Update the exact FileCheck syntax of the 256-bit and 512-bit
shuffle tests to match that used in the script I posted and now used
consistently in 128-bit tests.

Nothing interesting changing here, just using the label name as the
FileCheck label and a slightly more general comment marker consumption
strategy.

llvm-svn: 218709