[RegAllocFast] insert additional spills along indirect edges of INLINEASM_BR

When generating spills (stores) for values produced by INLINEASM_BR
instructions, make sure to insert one spill per indirect target.
Otherwise the reload generated may load from a stack slot that has not
yet been stored to (resulting in a load of an uninitialized stack slot).

Link: https://github.com/llvm/llvm-project/issues/53562
Fixes: https://github.com/llvm/llvm-project/issues/60855

Reviewed By: MatzeB

Differential Revision: https://reviews.llvm.org/D144907