[X86] LowerShiftByScalarImmediate - move shl(x,1) -> add(freeze(x),freeze(x)) to X86FixupInstTunings

[RKSimon]

Avoid the shl(x,1) -> add(freeze(x),freeze(x)) if the shift-imm is legal, and leave it to X86FixupInstTunings.

Helps avoid missed optimisations due to oneuse limits, avoids unnecessary freezes and allows AVX512 to fold to mi memory folding variants

[llvmbot]

@llvm/issue-subscribers-backend-x86

Author: Simon Pilgrim (RKSimon)

Avoid the shl(x,1) -> add(freeze(x),freeze(x)) if the shift-imm if legal, and leave it to X86FixupInstTunings.

Helps avoid missed optimisations due to oneuse limits, avoids unnecessary freezes and allows AVX512 to fold to mi memory folding variants