[x86] Fix a large collection of bugs that crept in as I fleshed out the
AVX support.

New test cases included. Note that none of the existing test cases
covered these buggy code paths. =/ Also, it is clear from this that
SHUFPS and SHUFPD are the most bug prone shuffle instructions in x86. =[

These were all detected by fuzz-testing. (I <3 fuzz testing.)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218522 91177308-0d34-0410-b5e6-96231b3b80d8