[CVP] Narrow SDiv/SRem to the smallest power-of-2 that's sufficient to contain its operands

This is practically identical to what we already do for UDiv/URem:
  https://rise4fun.com/Alive/04K

Name: narrow udiv
Pre: C0 u<= 255 && C1 u<= 255
%r = udiv i16 C0, C1
  =>
%t0 = trunc i16 C0 to i8
%t1 = trunc i16 C1 to i8
%t2 = udiv i8 %t0, %t1
%r = zext i8 %t2 to i16

Name: narrow exact udiv
Pre: C0 u<= 255 && C1 u<= 255
%r = udiv exact i16 C0, C1
  =>
%t0 = trunc i16 C0 to i8
%t1 = trunc i16 C1 to i8
%t2 = udiv exact i8 %t0, %t1
%r = zext i8 %t2 to i16

Name: narrow urem
Pre: C0 u<= 255 && C1 u<= 255
%r = urem i16 C0, C1
  =>
%t0 = trunc i16 C0 to i8
%t1 = trunc i16 C1 to i8
%t2 = urem i8 %t0, %t1
%r = zext i8 %t2 to i16

... only here we need to look for 'min signed bits', not 'active bits',
and there's an UB to be aware of:
  https://rise4fun.com/Alive/KG86
  https://rise4fun.com/Alive/LwR

Name: narrow sdiv
Pre: C0 <= 127 && C1 <= 127 && C0 >= -128 && C1 >= -128
%r = sdiv i16 C0, C1
  =>
%t0 = trunc i16 C0 to i9
%t1 = trunc i16 C1 to i9
%t2 = sdiv i9 %t0, %t1
%r = sext i9 %t2 to i16

Name: narrow exact sdiv
Pre: C0 <= 127 && C1 <= 127 && C0 >= -128 && C1 >= -128
%r = sdiv exact i16 C0, C1
  =>
%t0 = trunc i16 C0 to i9
%t1 = trunc i16 C1 to i9
%t2 = sdiv exact i9 %t0, %t1
%r = sext i9 %t2 to i16

Name: narrow srem
Pre: C0 <= 127 && C1 <= 127 && C0 >= -128 && C1 >= -128
%r = srem i16 C0, C1
  =>
%t0 = trunc i16 C0 to i9
%t1 = trunc i16 C1 to i9
%t2 = srem i9 %t0, %t1
%r = sext i9 %t2 to i16


Name: narrow sdiv
Pre: C0 <= 127 && C1 <= 127 && C0 >= -128 && C1 >= -128 && !(C0 == -128 && C1 == -1)
%r = sdiv i16 C0, C1
  =>
%t0 = trunc i16 C0 to i8
%t1 = trunc i16 C1 to i8
%t2 = sdiv i8 %t0, %t1
%r = sext i8 %t2 to i16

Name: narrow exact sdiv
Pre: C0 <= 127 && C1 <= 127 && C0 >= -128 && C1 >= -128 && !(C0 == -128 && C1 == -1)
%r = sdiv exact i16 C0, C1
  =>
%t0 = trunc i16 C0 to i8
%t1 = trunc i16 C1 to i8
%t2 = sdiv exact i8 %t0, %t1
%r = sext i8 %t2 to i16

Name: narrow srem
Pre: C0 <= 127 && C1 <= 127 && C0 >= -128 && C1 >= -128 && !(C0 == -128 && C1 == -1)
%r = srem i16 C0, C1
  =>
%t0 = trunc i16 C0 to i8
%t1 = trunc i16 C1 to i8
%t2 = srem i8 %t0, %t1
%r = sext i8 %t2 to i16


The ConstantRangeTest.losslessSignedTruncationSignext test sanity-checks
the logic, that we can losslessly truncate ConstantRange to
`getMinSignedBits()` and signext it back, and it will be identical
to the original CR.

On vanilla llvm test-suite + RawSpeed, this fires 1262 times,
while the same fold for UDiv/URem only fires 384 times. Sic!

Additionally, this causes +606.18% (+1079) extra cases of
aggressive-instcombine.NumDAGsReduced, and +473.14% (+1145)
of aggressive-instcombine.NumInstrsReduced folds.