The legacy llvm::identity is not quite the same as std::identity from
C++20.  llvm::identity is a template struct with an ::argument_type
member.  In contrast, llvm::identity_cxx20 (and std::identity) is a
non-template struct with a templated call operator and no
::argument_type.

This patch modernizes llvm::SparseSet by updating its default
key-extraction functor to llvm::identity_cxx20.  A new template
parameter KeyT takes over the role of ::argument_type.

Existing uses of SparseSet are updated for the new template signature.
Most use sites are of the form SparseSet<T>, requiring no update.

This is the fast regalloc equivalent of
773771ba382b1fbcf6acccc0046bfe731541a599.

Add per-property has<Prop>/set<Prop>/reset<Prop> functions to
MachineFunctionProperties.

We have already ensured in 9cec2b246e719533723562950e56c292fe5dd5ad
that `INLINEASM_BR` output operands get spilled onto the stack, both
in the fallthrough path and in the indirect targets. Since reloads of
live-ins values into physical registers contextually happen after all
MIR instructions (and ops) have been visited, make sure such loads are
placed at the start of the block, but after prologues or `INLINEASM_BR`
spills, as otherwise this may cause stale values to be read from the
stack.

Fixes: #74483, #110251.

Found by msan.
==8138==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x559016395beb in allocVirtRegUndef llvm/lib/CodeGen/RegAllocFast.cpp:1010:6

This reverts commit 5fadb3d680909ab30b37eb559f80046b5a17045e.