[InstCombine] and-mask + conditional "sext" -> and-mask + sext

[LebedevRI]

Bugzilla Link	42412
Version	trunk
OS	Linux
CC	@RKSimon,@nikic,@rotateright

Extended Description

There's also identical problem to #41734 with different bit buffer abstraction:
https://godbolt.org/z/po3_0Y
https://rise4fun.com/Alive/btQ

int bad(unsigned input, unsigned len) {
unsigned diff = input & ((1 << len)-1);
// If the first bit is 1 we need to turn this into a negative number
if (diff >> (len - 1))
diff -= (1 << len);
return diff;
}
int good(unsigned input, unsigned len) {
unsigned diff = input & ((1 << len)-1);
diff <<= (32-len);
diff = int(diff) >> (32-len);
return diff;
}

Name:
%o3 = shl i32 1, %len
%o4 = add nsw i32 %o3, -1
%o5 = and i32 %o4, %data
%o6 = add i32 %len, -1
%o7 = lshr i32 %o5, %o6
%o8 = icmp eq i32 %o7, 0
%o9 = select i1 %o8, i32 0, i32 %o3
%r = sub i32 %o5, %o9
=>
%n3 = shl nsw i32 -1, %len
%n4 = xor i32 %n3, -1
%n5 = and i32 %n4, %data
%n6 = sub i32 32, %len
%n7 = shl i32 %n5, %n6
%r = ashr i32 %n7, %n6

Did not analyse this one yet.