[ExpandLargeFpConvert] Fix incorrect values in fp-to-int conversion. (#86514)

The IR for a double-to-i129 conversion looks like this in one of the
blocks in compiler-rt:

  %cmp5.i = icmp ult i16 %3, -129, !dbg !24

But in ExpandLargeFpConvert, it looks like:

  %13 = icmp ult i129 %12, 4294967167, !dbg !19

ExpandLargeFpConvert is wrong; the value should have been
signed before negating, but instead we get a very large
unsigned value. Another value in the same pass also has this
issue.