PR21101: tablegen's FastISel emitter should filter out unused functions.

FastISel has a fixed set of virtual functions that are overridden by the
tablegen-generated code for each target. These functions are distinguished by
the kinds of operands, e.g., register + immediate = "ri". The FastISel emitter
has been blindly emitting functions with different combinations of operand
kinds, even for combinations that are completely unused by FastISel, e.g.,
"fastEmit_rrr". Change to filter out functions that will be irrelevant for
FastISel and do not bother generating the code for them. Also add explicit
"override" keywords for the virtual functions that are overridden.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218838 91177308-0d34-0410-b5e6-96231b3b80d8
[x86] Improve and correct how the new vector shuffle lowering was
matching and lowering 64-bit insertions.

The first problem was that we weren't looking through bitcasts to
discover that we *could* lower as insertions. Once fixed, we in turn
weren't looking through bitcasts to discover that we could fold a load
into the lowering. Once fixed, we weren't forming a SCALAR_TO_VECTOR
node around the inserted element and instead were passing a scalar to
a DAG node that expected a vector. It turns out there are some patterns
that will "lower" this into the correct asm, but the rest of the X86
backend is very unhappy with such antics.

This should fix a few more edge case regressions I've spotted going
through the regression test suite to enable the new vector shuffle
lowering.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218839 91177308-0d34-0410-b5e6-96231b3b80d8