[X86] Improve shift combining

This folds (ashr (shl a, [56,48,32,24,16]), SarConst)
into       (shl, (sext (a), [56,48,32,24,16] - SarConst))
or into    (lshr, (sext (a), SarConst - [56,48,32,24,16]))
depending on sign of (SarConst - [56,48,32,24,16])

sexts in X86 are MOVs.
The MOVs have the same code size as above SHIFTs (only SHIFT by 1 has lower code size).
However the MOVs have 2 advantages to SHIFTs on x86:
1. MOVs can write to a register that differs from source.
2. MOVs accept memory operands.

This fixes PR24373.

Patch by: evgeny.v.stupachenko@intel.com
Differential Revision: http://reviews.llvm.org/D13161

llvm-svn: 255761