Automated with clang-tidy -fix -checks=-*,modernize-use-default-member-init

Outside of CodeGenRegisters itself, we only want to find existing
SubRegIdxs, not create new ones. Change findSubRegIdx to assert and use
it consistently for this purpose.

- Change `Name`, `SubopNames`, `PrinterMethodName`, and
`EncoderMethodNames` to be stored as StringRef.
- Also changed `CheckComplexPatMatcher::Name` to StringRef as a fallout
from the above.

Verified that all the tablegen generated files within LLVM are
unchanged.

- Also eliminate unneeded std::string() around some literal strings.

Also use brace initialization and emplace to avoid explicitly 
constructing std::pair, and the same for std::tuple.

Print what result number the Emit* nodes are storing their results in.
This makes it easy to track the inputs of later opcodes that consume
these results.

The node was introduced in 59c39dc1 and was intended to allow writing
patterns like this:
`[(set AL, (mul AL, GR8:$src1)), (implicit EFLAGS)]`

However, it does not introduce new functionality because the same
pattern can be equivalently expressed as:
`[(set AL, EFLAGS, (mul AL, GR8:$src1))]`

The latter form is also more flexible as it allows reordering output
operands.

In most places uses of `implicit` were redundant -- removing them didn't
change anything in the generated DAG tables. The only three cases where
it did have effect are in X86InstrArithmetic.td and X86InstrSystem.td --
those were rewritten to use `set` node.

Removing `implicit` from some patterns made them importable by GISel,
hence the change in a test.