Cmm sinking does not eliminate dead code in loops

Cmm code shown in #8326 is suboptimal. For this Haskell code:

isTrue# :: Int# -> Bool
isTrue# 1# = True
isTrue# _  = False

f :: Int# -> Int
f x | isTrue# (x ># 0#) = I# x
    | otherwise         = -(I# x)

we get Cmm that contains dead code in a loop:

       cFg: // False branch
           Hp = Hp + 16;
           if (Hp > HpLim) goto cFy; else goto cFx;
       cFy: // not enough heap, call GC
           HpAlloc = 16;
           I64[Sp - 16] = cFf;
           R1 = _sEV::I64;
           I64[Sp - 8] = _sEU::I64;
           Sp = Sp - 16;
           call stg_gc_unbx_r1(R1) returns to cFf, args: 8, res: 8, upd: 8;
       cFf: // re-do the False branch
           _sEU::I64 = I64[Sp + 8];
           Sp = Sp + 16;
           _sEV::I64 = R1;
           goto cFg;
       cFx: // RHS of False branch
           I64[Hp - 8] = GHC.Types.I#_con_info;
           I64[Hp] = -_sEU::I64;
           R1 = Hp - 7;
           call (P64[Sp])(R1) args: 8, res: 0, upd: 8;

Notice that _sEV is dead - we should optimize it away. This is a known deficiency in the Cmm sinking pass (i.e. it does not work for loops). I'm putting this on Trac so we have this documented.

Trac metadata

Trac field	Value
Version	7.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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