Make sure all allocas are created in the entry block and are given the
right lifetimes. This is good for code quality:
* Moving allocas to the entry block makes sure they are always
allocated statically (avoiding the need for a frame pointer) and do
not grow the stack on each new alloca instruction. This is
especially useful in loops where it could otherwise lead to a stack
overflow even though there is no recursion.
* Adding lifetime markers allows LLVM to reuse stack areas for
different allocas as long as their lifetimes do not overlap.
All in all, this reduces code size in all tested cases for the BBC
micro:bit, and reduces code size for most cases for WebAssembly.
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