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

In the legacy space, if both the 66 prefix and REX.W=1 are present, the
REX.W=1 takes precedence and makes OSIZE=64b. EVEX map 4 inherits this
convention, with EVEX.pp=01 and EVEX.W playing the roles of the 66
prefix and REX.W. So if EVEX.pp=00, the OSIZE can only be 64b or 32b,
depending on whether EVEX.W=1 or not. But if EVEX.pp=01, then OSIZE is
either 64b or 16b depending on whether EVEX.W=1 or not.

Ref.: https://cdrdv2.intel.com/v1/dl/getContent/828965

Ref.: https://cdrdv2.intel.com/v1/dl/getContent/828965

Reverts llvm/llvm-project#101452

There are several buildbot failed. Revert first.

Ref.: https://cdrdv2.intel.com/v1/dl/getContent/828965

Since `raw_string_ostream` doesn't own the string buffer, it is
desirable (in terms of memory safety) for users to directly reference
the string buffer rather than use `raw_string_ostream::str()`.

Work towards TODO item to remove `raw_string_ostream::str()`.

Extended VMX instructions and 8 bit apx extended instructions don't need
W bit, they are marked as W ignored in spec.
RFC:
https://discourse.llvm.org/t/rfc-design-for-apx-feature-egpr-and-ndd-support/73031/4