[flang] Fix a bug with simplified minloc that treated logicals with even values > 1 as 0

Previously the mask would be loaded as the appropriate integer type and cast to I1 to pass to
fir.if, however this truncates the integer and so would cast 6 to 0. By loading values as logicals
and casting to I1 this problem is avoided.

Reviewed By: Leporacanthicus

Differential Revision: https://reviews.llvm.org/D144974