[InstCombine] narrow width of rotate patterns, part 2 (PR39624)

The sub-pattern for the shift amount in a rotate can take on
several different forms, and there's apparently no way to
canonicalize those without seeing the entire rotate sequence.

This is the form noted in:
https://bugs.llvm.org/show_bug.cgi?id=39624

https://rise4fun.com/Alive/qnT

  %zx = zext i8 %x to i32
  %maskedShAmt = and i32 %shAmt, 7
  %shl = shl i32 %zx, %maskedShAmt
  %negShAmt = sub i32 0, %shAmt
  %maskedNegShAmt = and i32 %negShAmt, 7
  %shr = lshr i32 %zx, %maskedNegShAmt
  %rot = or i32 %shl, %shr
  %r = trunc i32 %rot to i8
  =>
  %truncShAmt = trunc i32 %shAmt to i8
  %maskedShAmt2 = and i8 %truncShAmt, 7
  %shl2 = shl i8 %x, %maskedShAmt2
  %negShAmt2 = sub i8 0, %truncShAmt
  %maskedNegShAmt2 = and i8 %negShAmt2, 7
  %shr2 = lshr i8 %x, %maskedNegShAmt2
  %r = or i8 %shl2, %shr2

llvm-svn: 346713