[LTT] Handle merged llvm.assume when dropping type tests

When the lower type test pass is invoked a second time with
DropTypeTests set to true, it expects that all remaining type tests feed
assume instructions, which are removed along with the type tests.

In some cases the llvm.assume might have been merged with another one,
i.e. from a builtin_assume instruction, in which case the type test
would actually feed a phi that in turn feeds the merged assume
instruction. In this case we can simply replace that operand of the phi
with "true" before removing the type test.

Differential Revision: https://reviews.llvm.org/D103073