Draft: add support for conditional assignment (branchless code) (fix #19448)

This MR adds support for branchless assignments in Cmm and in the X86 ncg backend. Fix #19448.

• Pass testsuite
• Benchmark performance impact of the change: ~15% performance improvement in a benchmark)
• Write a note: see Note[Conditional assignments] in GHC.StgToCmm.Expr
• Add support for DataAlt. E.g.

foo :: Bool -> Int
foo x = case x of
  True -> 10
  False -> 27

• Add testcase for DataAlt case
• Fix #21710 (closed) or avoid using DataToTagOp in DataAlt case