[InstCombine] Evaluate new shift amount for sext(ashr(shl(trunc()))) fold in wide type (PR48343)

It is not correct to compute that new shift amount in it's narrow type
and only then extend it into the wide type:

----------------------------------------
Optimization: PR48343 good
Precondition: (width(%X) == width(%r))
  %o0 = trunc %X
  %o1 = shl %o0, %Y
  %o2 = ashr %o1, %Y
  %r = sext %o2
=>
  %n0 = sext %Y
  %n1 = sub width(%o0), %n0
  %n2 = sub width(%X), %n1
  %n3 = shl %X, %n2
  %r = ashr %n3, %n2

Done: 2016
Optimization is correct!

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Optimization: PR48343 bad
Precondition: (width(%X) == width(%r))
  %o0 = trunc %X
  %o1 = shl %o0, %Y
  %o2 = ashr %o1, %Y
  %r = sext %o2
=>
  %n0 = sub width(%o0), %Y
  %n1 = sub width(%X), %n0
  %n2 = sext %n1
  %n3 = shl %X, %n2
  %r = ashr %n3, %n2

Done: 1
ERROR: Domain of definedness of Target is smaller than Source's for i9 %r

Example:
%X i9 = 0x000 (0)
%Y i4 = 0x3 (3)
%o0 i4 = 0x0 (0)
%o1 i4 = 0x0 (0)
%o2 i4 = 0x0 (0)
%n0 i4 = 0x1 (1)
%n1 i4 = 0x8 (8, -8)
%n2 i9 = 0x1F8 (504, -8)
%n3 i9 = 0x000 (0)
Source value: 0x000 (0)
Target value: undef


I.e. we should be computing it in the wide type from the beginning.

Fixes https://bugs.llvm.org/show_bug.cgi?id=48343